block: fix missing bio back/front segment size setting in blk_recount_segments()

block: fix missing bio back/front segment size setting in blk_recount_segments()

Commit 29bd6f25747fedb54393e6304ddf33f041218643 introduced a bug where we
don't get front/back segment sizes in the bio in blk_recount_segments().
Fix this by tracking the back bio as well as the front bio in
__blk_recalc_rq_segments(), this also cleans up the interface by getting
rid of the segment size pointer passing.

Tested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>