nbd: fix crash when the blksize is zero

[ Upstream commit 553768d1169a48c0cd87c4eb4ab57534ee663415 ]

This will allow the blksize to be set zero and then use 1024 as
default.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
[fix to use goto out instead of return in genl_connect]
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>