libxfs: expose inobtcount in xfs geometry

As xfs supports the feature of inode btree block counters now, expose
this feature flag in xfs geometry, for userspace can check if the
inobtcnt is enabled or not.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>