tcp: Factorise cookie-independent fields initialisation in cookie_v[46]_check().

We will support arbitrary SYN Cookie with BPF, and then some reqsk fields
are initialised in kfunc, and others are done in cookie_v[46]_check().

This patch factorises the common part as cookie_tcp_reqsk_init() and
calls it in cookie_tcp_reqsk_alloc() to minimise the discrepancy between
cookie_v[46]_check().

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/20231129022924.96156-8-kuniyu@amazon.com


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