xfs: store inode btree block counts in AGI header

Add a btree block usage counters for both inode btrees to the AGI header
so that we don't have to walk the entire finobt at mount time to create
the per-AG reservations.

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