fs/xattr: Fix handling of AT_FDCWD in setxattrat(2) and getxattrat(2)

Currently, setxattrat(2) and getxattrat(2) are wrongly handling the
calls of the from setxattrat(AF_FDCWD, NULL, AT_EMPTY_PATH, ...) and
fail with -EBADF error instead of operating on CWD. Fix it.

Fixes: 6140be90 ("fs/xattr: add *at family syscalls")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/20250424132246.16822-2-jack@suse.cz


Signed-off-by: Christian Brauner <brauner@kernel.org>