xfs: introduce new v5 bulkstat structure

Introduce a new version of the in-core bulkstat structure that supports
our new v5 format features.  This structure also fills the gaps in the
previous structure.  We leave wiring up the ioctls for the next patch.

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