block: Add warning for bi_next not NULL in bio_endio()

block: Add warning for bi_next not NULL in bio_endio()

Recently found a bug where a driver left bi_next not NULL and then
called bio_endio(), and then the submitter of the bio used
bio_copy_data() which was treating src and dst as lists of bios.

Fixed that bug by splitting out bio_list_copy_data(), but in case other
things are depending on bi_next in weird ways, add a warning to help
avoid more bugs like that in the future.

Signed-off-by: Kent Overstreet <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>