ublk: move check for empty address field on command submission

In preparation for zoned storage support, move the check for empty `addr`
field into the command handler case statement. Note that the check makes no
sense for `UBLK_IO_NEED_GET_DATA` because the `addr` field must always be
set for this command.

Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230804114610.179530-3-nmi@metaspace.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>