nbd: clean up return value checking of sock_xmit()

mainline inclusion
from mainline-next-20211018
commit 6157a8f489909db00151a4e361903b9099b03b75
category: bugfix
bugzilla: 49890
CVE: NA
---------------------------

Check if sock_xmit() return 0 is useless because it'll never return
0, comment it and remove such checkings.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210916093350.1410403-6-yukuai3@huawei.com


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

Conflict:drivers/block/nbd.c
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>