fs/xfs: Fix grub_xfs_iterate_dir() return value in case of failure

fs/xfs: Fix grub_xfs_iterate_dir() return value in case of failure

Commit ef7850c757 (fs/xfs: Fix issues found while fuzzing the XFS
filesystem) introduced multiple boundary checks in grub_xfs_iterate_dir()
but handled the error incorrectly returning error code instead of 0.
Fix it. Also change the error message so that it doesn't match the
message in grub_xfs_read_inode().

Fixes: ef7850c757 (fs/xfs: Fix issues found while fuzzing the XFS filesystem)
Signed-off-by: Egor Ignatov <egori@altlinux.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>