ocfs2: convert to new aops

Plug ocfs2 into the ->write_begin and ->write_end aops.

A bunch of custom code is now gone - the iovec iteration stuff during write
and the ocfs2 splice write actor.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>