On low LUN configurations, make sure not to send bios that are bigger
than the buffer size.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: NJavier González <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>