xfs: rename xchk_get_inode -> xchk_iget_for_scrubbing

Dave Chinner suggested renaming this function to make more obvious what
it does.  The function returns an incore inode to callers that want to
scrub a metadata structure that hangs off an inode.  If the iget fails
with EINVAL, it will single-step the loading process to distinguish
between actually free inodes or impossible inumbers (ENOENT);
discrepancies between the inobt freemask and the free status in the
inode record (EFSCORRUPTED).  Any other negative errno is returned
unchanged.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>