statx: Kill fd-with-NULL-path support in favour of AT_EMPTY_PATH

With the new statx() syscall, the following both allow the attributes of
the file attached to a file descriptor to be retrieved:

statx(dfd, NULL, 0, ...);

and:

statx(dfd, "", AT_EMPTY_PATH, ...);

Change the code to reject the first option, though this means copying
the path and engaging pathwalk for the fstat() equivalent. dfd can be a
non-directory provided path is "".

[ The timing of this isn't wonderful, but applying this now before we
have statx() in any released kernel, before anybody starts using the
NULL special case. - Linus ]

Fixes: a528d35e8bfc ("statx: Add a system call to make enhanced file info available")
Reported-by: Michael Kerrisk <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
cc: Eric Sandeen <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

fs/stat.c

···
 /**
  * sys_statx - System call to get enhanced stats
  * @dfd: Base directory to pathwalk from *or* fd to stat.
- * @filename: File to stat *or* NULL.
+ * @filename: File to stat or "" with AT_EMPTY_PATH
  * @flags: AT_* flags to control pathwalk.
  * @mask: Parts of statx struct actually required.
  * @buffer: Result buffer.
  *
- * Note that if filename is NULL, then it does the equivalent of fstat() using
- * dfd to indicate the file of interest.
+ * Note that fstat() can be emulated by setting dfd to the fd of interest,
+ * supplying "" as the filename and setting AT_EMPTY_PATH in the flags.
  */
 SYSCALL_DEFINE5(statx,
 		int, dfd, const char __user *, filename, unsigned, flags,
···
 		return -EINVAL;
 	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
 		return -EINVAL;
+	if (!filename)
+		return -EINVAL;
 
-	if (filename)
-		error = vfs_statx(dfd, filename, flags, &stat, mask);
-	else
-		error = vfs_statx_fd(dfd, &stat, mask, flags);
+	error = vfs_statx(dfd, filename, flags, &stat, mask);
 	if (error)
 		return error;