xfs: remove the unused ic_io_size field from xlog_in_core

ic_io_size is only used inside xlog_write_iclog, where we can just use
the count parameter intead.

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