xfs: support inode btree blockcounts in online repair

Add the necessary bits to the online repair code to support logging the
inode btree counters when rebuilding the btrees, and to support fixing
the counters when rebuilding the AGI.

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