btrfs: Put mirror_num in bi_bdev

The error correction code wants to make sure that only the bad mirror is
rewritten. Thus, we need to know which mirror is the bad one. I did not
find a more apropriate field than bi_bdev. But I think using this is fine,
because it is modified by the block layer, anyway, and should not be read
after the bio returned.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>