xfs: report finobt status in fs geometry

Define the XFS_FSOP_GEOM_FLAGS_FINOBT fs geometry flag and set the
associated bit if the filesystem supports the free inode btree.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>