ocfs2: report error from o2hb_do_disk_heartbeat() to user

Report return value of o2hb_do_disk_heartbeat() as a part of ML_HEARTBEAT
message so that we know whether a heartbeat actually happened or not.
This also makes assigned but otherwise unused 'ret' variable used.

Coverity id: 1227053.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>