nbd: null check for nla_nest_start

nbd: null check for nla_nest_start

nla_nest_start() may fail and return NULL. Insert a check and set errno
based on other call sites within the same source code.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Fixes: 47d902b90a32 ("nbd: add a status netlink command")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240218042534.it.206-kees@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>