ocfs2: submit disk heartbeat bio using WRITE_SYNC

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 at jp.fujitsu.com>
Acked-by: Tao Ma <boyu.mt at taobao.com>
Acked-by: Sunil Mushran <sunil.mushran at oracle.com>
Signed-off-by: Srinivas Eeda <srinivas.eeda@oracle.com>