ocfs2/trivial: Use proper mask for 2 places in hearbeat.c

I just noticed today that there are 2 places of "mlog(0,...)"
in  fs/ocfs2/cluster/heartbeat.c, but actually have no default
mask prefix in that file.
So change them to mlog(ML_HEARTBEAT,...).

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>