xfs: clean up calculation of LR header blocks

Let's use DIV_ROUND_UP() to calculate log record header
blocks as what did in xlog_get_iclog_buffer_size() and
wrap up a common helper for log recovery.

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>