xfs: fix parent pointer scrubber bailing out on unallocated inodes

xfs_iget can return -ENOENT for a file that the inobt thinks is
allocated but has zeroed mode.  This currently causes scrub to exit
with an operational error instead of flagging this as a corruption.  The
end result is that scrub mistakenly reports the ENOENT to the user
instead of "directory parent pointer corrupt" like we do for EINVAL.

Fixes: 5927268f ("xfs: flag inode corruption if parent ptr doesn't get us a real inode")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>