statx: correct error handling of NULL pathname

The change in commit 1e2f82d1e9d1 ("statx: Kill fd-with-NULL-path
support in favour of AT_EMPTY_PATH") to error on a NULL pathname to
statx() is inconsistent.

It results in the error EINVAL for a NULL pathname. Other system calls
with similar APIs (fchownat(), fstatat(), linkat()), return EFAULT.

The solution is simply to remove the EINVAL check. As I already pointed
out in [1], user_path_at*() and filename_lookup() will handle the NULL
pathname as per the other APIs, to correctly produce the error EFAULT.

[1] https://lkml.org/lkml/2017/4/26/561

Signed-off-by: Michael Kerrisk <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Eric Sandeen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/stat.c

···
 		return -EINVAL;
 	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
 		return -EINVAL;
-	if (!filename)
-		return -EINVAL;
 
 	error = vfs_statx(dfd, filename, flags, &stat, mask);
 	if (error)