ocfs2: Concurrent access of o2hb_region->hr_task was not locked

This means that a build-up and a teardown could race which would result in a
double-kthread_stop().

Protect the setting and clearing of hr_task with o2hb_live_lock, as it's not
a common thing and not performance critical.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>