gfs2: Do not use atomic operations unnecessarily

The GLF_DEMOTE_IN_PROGRESS and GLF_LOCK flags and the glock refcount are
all protected by the glock spin lock, so there is no need for atomic
operations / barriers here.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>