RDMA/core: Ensure that rdma_user_mmap_entry_remove() is a fence

The set of entry->driver_removed is missing locking, protect it with
xa_lock() which is held by the only reader.

Otherwise readers may continue to see driver_removed = false after
rdma_user_mmap_entry_remove() returns and may continue to try and
establish new mmaps.

Fixes: 3411f9f01b76 ("RDMA/core: Create mmap database and cookie helper functions")
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Gal Pressman <[e-mail]>
Acked-by: Michal Kalderon <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/core/ib_core_uverbs.c

···
 	if (!entry)
 		return;
 
+	xa_lock(&entry->ucontext->mmap_xa);
 	entry->driver_removed = true;
+	xa_unlock(&entry->ucontext->mmap_xa);
 	kref_put(&entry->ref, rdma_user_mmap_entry_free);
 }
 EXPORT_SYMBOL(rdma_user_mmap_entry_remove);