block: Split out bio_list_copy_data()

Found a bug (with ASAN) where we were passing a bio to bio_copy_data()
with bi_next not NULL, when it should have been - a driver had left
bi_next set to something after calling bio_endio().

Since the normal case is only copying single bios, split out
bio_list_copy_data() to avoid more bugs like this in the future.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>