ocfs2: Add an incompat feature flag OCFS2_FEATURE_INCOMPAT_CLUSTERINFO

OCFS2_FEATURE_INCOMPAT_CLUSTERINFO allows us to use sb->s_cluster_info for
both userspace and o2cb cluster stacks. It also allows us to extend cluster
info to include stack flags.

This patch also adds stackflags to sb->s_clusterinfo. It also introduces a
clusterinfo flag OCFS2_CLUSTER_O2CB_GLOBAL_HEARTBEAT to denote the enabled
global heartbeat mode.

This incompat flag can be set/cleared using tunefs.ocfs2 --fs-features. The
clusterinfo flag is set/cleared using tunefs.ocfs2 --update-cluster-stack.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>