Introductory patches for bio refactor.

Hi Jens,
 I wonder if you would accept these patches the block layer.
 They are, as far as I can tell, quite uncontroversial and provide
 good cleanups.

 The first is a minor bug-fix.
 The next to replace helper function that take a bio (always the first
 bio of a request), to instead take a request.

 The last two combine to replace rq_for_each_bio with
 rq_for_each_segment which makes blk_recalc_rq_segments a lot
 cleaner, and improves (to a lesser degree) every other place that
 used rq_for_each_bio.

 The net result is a decrease in object code size (for my x86-64 compile)
 for block/built-in.o of 96 bytes, though there is some growth out in
 drivers making and over-all decrease of only 48 bytes.

Thanks,
NeilBrown



 [PATCH 001 of 5] Don't update bi_hw_*_size if we aren't going to merge.
 [PATCH 002 of 5] Replace bio_data with blk_rq_data
 [PATCH 003 of 5] Replace bio_cur_sectors with blk_rq_cur_sectors.
 [PATCH 004 of 5] Introduce rq_for_each_segment replacing rq_for_each_bio
 [PATCH 005 of 5] Merge blk_recount_segments into blk_recalc_rq_segments