ocfs2: Flush drive's caches on fdatasync

When 'barrier' mount option is specified, we have to issue a cache flush
during fdatasync(2). We have to do this even if inode doesn't have
I_DIRTY_DATASYNC set because we still have to get written *data* to disk so
that they are not lost in case of crash.

Acked-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Singed-off-by: Tao Ma <tao.ma@oracle.com>