tcp: move sysctl_tcp_shrink_window to netns_ipv4_read_txrx group

tcp: move sysctl_tcp_shrink_window to netns_ipv4_read_txrx group

Commit 18fd64d25422 ("netns-ipv4: reorganize netns_ipv4 fast path
variables") missed that __tcp_select_window() is reading
net->ipv4.sysctl_tcp_shrink_window.

Move this field to netns_ipv4_read_txrx group, as __tcp_select_window()
is used both in tx and rx paths.

Saves a potential cache line miss.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260307092214.2433548-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>