netfilter: use actual socket sk for REJECT action

True to the message of commit v5.10-rc1-105-g46d6c5ae953c, _do_
actually make use of state->sk when possible, such as in the REJECT
modules.

Reported-by: Minqiang Chen <ptpt52@gmail.com>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>