ocfs2: Wake up a starting region if it gets killed in the background.

Tell o2cb_region_dev_write() to wake up if rmdir(2) happens on the
heartbeat region while it is starting up.  Then o2hb_region_dev_write()
can check to see if it is alive and act accordingly.  This prevents a hang
(not being woken) and a crash (if it's woken by a signal).

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