tcp: Cache sock_net(sk) in cookie_v[46]_check().

sock_net(sk) is used repeatedly in cookie_v[46]_check().
Let's cache it in a variable.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231129022924.96156-3-kuniyu@amazon.com


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