ublk: don't mutate struct bio_vec in iteration

ublk: don't mutate struct bio_vec in iteration

__bio_for_each_segment() uses the returned struct bio_vec's bv_len field
to advance the struct bvec_iter at the end of each loop iteration. So
it's incorrect to modify it during the loop. Don't assign to bv_len (or
bv_offset, for that matter) in ublk_copy_user_pages().

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: e87d66ab27ac ("ublk: use rq_for_each_segment() for user copy")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>