ocfs2: Update syncing after splicing to match generic version

Update ocfs2 specific splicing code to use generic syncing helper. The sync now
does not happen under rw_lock because generic_write_sync() acquires i_mutex
which ranks above rw_lock. That should not matter because standard fsync path
does not hold it either.

Acked-by: Joel Becker <Joel.Becker@oracle.com>
Acked-by: Mark Fasheh <mfasheh@suse.com>
CC: ocfs2-devel@oss.oracle.com
Signed-off-by: Jan Kara <jack@suse.cz>