tcp: increase tcp_rmem[2] to 32 MB

Last change to tcp_rmem[2] happened in 2012, in commit b49960a0
("tcp: change tcp_adv_win_scale and tcp_rmem[2]")

TCP performance on WAN is mostly limited by tcp_rmem[2] for receivers.

After this series improvements, it is time to increase the default.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-12-edumazet@google.com


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