xarray: Document necessary flag in alloc functions

Adds a new line to the docstrings of functions wrapping __xa_alloc() and
__xa_alloc_cyclic(), informing about the necessity of flag XA_FLAGS_ALLOC
being set previously.

The documentation so far says that functions wrapping __xa_alloc() and
__xa_alloc_cyclic() are supposed to return either -ENOMEM or -EBUSY in
case of an error. If the xarray has been initialized without the flag
XA_FLAGS_ALLOC, however, they fail with a different, undocumented error
code.

As hinted at in Documentation/core-api/xarray.rst, wrappers around these
functions should only be invoked when the flag has been set. The
functions' documentation should reflect that as well.

Signed-off-by: Philipp Stanner <[e-mail]>
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>

include/linux/xarray.h

···
  * stores the index into the @id pointer, then stores the entry at
  * that index.  A concurrent lookup will not see an uninitialised @id.
  *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
+ *
  * Context: Any context.  Takes and releases the xa_lock.  May sleep if
  * the @gfp flags permit.
  * Return: 0 on success, -ENOMEM if memory could not be allocated or
···
  * stores the index into the @id pointer, then stores the entry at
  * that index.  A concurrent lookup will not see an uninitialised @id.
  *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
+ *
  * Context: Any context.  Takes and releases the xa_lock while
  * disabling softirqs.  May sleep if the @gfp flags permit.
  * Return: 0 on success, -ENOMEM if memory could not be allocated or
···
  * Finds an empty entry in @xa between @limit.min and @limit.max,
  * stores the index into the @id pointer, then stores the entry at
  * that index.  A concurrent lookup will not see an uninitialised @id.
+ *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
  *
  * Context: Process context.  Takes and releases the xa_lock while
  * disabling interrupts.  May sleep if the @gfp flags permit.
···
  * that index.  A concurrent lookup will not see an uninitialised @id.
  * The search for an empty entry will start at @next and will wrap
  * around if necessary.
+ *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
  *
  * Context: Any context.  Takes and releases the xa_lock.  May sleep if
  * the @gfp flags permit.
···
  * The search for an empty entry will start at @next and will wrap
  * around if necessary.
  *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
+ *
  * Context: Any context.  Takes and releases the xa_lock while
  * disabling softirqs.  May sleep if the @gfp flags permit.
  * Return: 0 if the allocation succeeded without wrapping.  1 if the
···
  * that index.  A concurrent lookup will not see an uninitialised @id.
  * The search for an empty entry will start at @next and will wrap
  * around if necessary.
+ *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
  *
  * Context: Process context.  Takes and releases the xa_lock while
  * disabling interrupts.  May sleep if the @gfp flags permit.

lib/xarray.c

···
  * stores the index into the @id pointer, then stores the entry at
  * that index.  A concurrent lookup will not see an uninitialised @id.
  *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
+ *
  * Context: Any context.  Expects xa_lock to be held on entry.  May
  * release and reacquire xa_lock if @gfp flags permit.
  * Return: 0 on success, -ENOMEM if memory could not be allocated or
···
  * that index.  A concurrent lookup will not see an uninitialised @id.
  * The search for an empty entry will start at @next and will wrap
  * around if necessary.
+ *
+ * Must only be operated on an xarray initialized with flag XA_FLAGS_ALLOC set
+ * in xa_init_flags().
  *
  * Context: Any context.  Expects xa_lock to be held on entry.  May
  * release and reacquire xa_lock if @gfp flags permit.