sock: add sock_kmemdup helper

This patch adds the sock version of kmemdup() helper, named sock_kmemdup(),
to duplicate the input "src" memory block using the socket's option memory
buffer.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/f828077394c7d1f3560123497348b438c875b510.1740735165.git.tanggeliang@kylinos.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>