null_blk: fix zoned support for non-rq based operation

The supported added for zones in null_blk seem to assume that only rq
based operation is possible. But this depends on the queue_mode setting,
if this is set to 0, then cmd->bio is what we need to be operating on.
Right now any attempt to load null_blk with queue_mode=0 will
insta-crash, since cmd->rq is NULL and null_handle_cmd() assumes it to
always be set.

Make the zoned code deal with bio's instead, or pass in the
appropriate sector/nr_sectors instead.

Fixes: ca4b2a01

 ("null_blk: add zone support")
Tested-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>