[rfc][patches] remove ->prepare_write

[Nick Piggin <npiggin@suse.de>]

Hi,

These are a set of patches to convert the last few filesystems to use
the new deadlock-free write aops, and remove the core code to handle the
legacy write path.

I don't really have setups to sufficiently test these filesystems. So I
would really appreciate if filesystem maintainers can pick these patches
up, bear with my bugs, and send them upstream when they're ready.

The benefit to you is that you get to use the fast and well tested code 
paths! Actually, it is interesting: several of the conversions I've done
(including these) take a relatively naive aproach of simply prefilling
the whole page if it isn't uptodate. It might be the case that some
filesystems actually prefer to do something similar to the legacy
double-copy path which they're being converted away from! (then again,
it would be probably even more ideal to have simple sub-page state
tracking structures).

There is still quite a lot of work left to be done. Several filesystems
still use prepare_write() helpers, and when they're fixed up, all the
old helpers themselves have to be removed. But this step is probably
most important to getting rid of complex code.