ocfs2: submit disk heartbeat bio using WRITE_SYNC

Under heavy I/O load, writing the disk heartbeat can be forced to wait for
minutes, and this causes the node to be fenced.

This patch tries to use WRITE_SYNC in submitting the heartbeat bio, so
that writing the heartbeat will have a priority over other requests.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
Acked-by: Tao Ma <tm@tao.ma>
Acked-by: Sunil Mushran <sunil.mushran@gmail.com>
Cc: Srinivas Eeeda <srinivas.eeda@oracle.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Tested-by: Gurudas Pai <gurudas.pai@oracle.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>