xfs: refactor eofb matching into a single helper

Refactor the two eofb-matching logics into a single helper so that we
don't repeat ourselves.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>