Make bio->bi_end_io() take bytes_done and actual error as argument. This
enables partial completion of bio's, which is important for latency
reasons (bio can be huge, for slow media we want page-by-page
completions).

I think I got most of the bi_end_io() functions out there, but I might
have missed a few. For the record, if you don't care about partial
completions and just want to be notified when the entire bio completes,
add a

	if (bio->bi_size)
		return 1;

to the top of your bi_end_io(). It should return 0 on completion.
bio_endio() will decrement bio->bi_size appropriately, it's recommended
for people to go through that. Otherwise they will have to control
BIO_UPTODATE and bi_size decrement themselves, there's really no reason
to do that. I've deliberately avoided doing any functional changes to
any of the end_io functions, as I think that would only make the patch
more complex. It's simple right now, but this being i/o paths I prefer
(as usual) to be careful and take small steps. The mpage_end_io_read()
do-vecs-at-the-time change can come right after this, for instance.