ocfs2: Use __generic_file_aio_write instead of generic_file_aio_write_nolock

Use the new helper. We have to submit data pages ourselves in case of O_SYNC
write because __generic_file_aio_write does not do it for us. OCFS2 developpers
might think about moving the sync out of i_mutex which seems to be easily
possible but that's out of scope of this patch.

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