xfs: update incore per-AG inode count

For whatever reason we never actually update pagi_count (the in-core
perag inode count) when we allocate or free inode chunks.  Online scrub
is going to use it, so we need to fix the accounting.

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