xfs: add rmap btree geometry feature flag

Originally-From: Dave Chinner <dchinner@redhat.com>

So xfs_info and other userspace utilities know the filesystem is
using this feature.

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