Fix various abuse of bio fields in umem.c

umem.c:
  advances bi_idx and bi_sector to track where it is up to.
   But it is only ever doing this on one bio, so the updated
   fields can easily be kept elsewhere (current_*).
  updates bi_size, but never uses the updated values, so
   this isn't needed.
  reuses bi_phys_segments to count how many iovecs have been
   completely.  As the completion happens sequentiually, we
   can store this information outside the bio too.

Signed-off-by: Neil Brown <neilb@suse.de>

diff .prev/drivers/block/umem.c ./drivers/block/umem.c
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>