net: set net.core.rmem_max and net.core.wmem_max to 4 MB

net: set net.core.rmem_max and net.core.wmem_max to 4 MB

SO_RCVBUF and SO_SNDBUF have limited range today, unless
distros or system admins change rmem_max and wmem_max.

Even iproute2 uses 1 MB SO_RCVBUF which is capped by
the kernel.

Decouple [rw]mem_max and [rw]mem_default and increase
[rw]mem_max to 4 MB.

Before:

$ sysctl net.core.rmem_default net.core.rmem_max net.core.wmem_default net.core.wmem_max
net.core.rmem_default = 212992
net.core.rmem_max = 212992
net.core.wmem_default = 212992
net.core.wmem_max = 212992

After:

$ sysctl net.core.rmem_default net.core.rmem_max net.core.wmem_default net.core.wmem_max
net.core.rmem_default = 212992
net.core.rmem_max = 4194304
net.core.wmem_default = 212992
net.core.wmem_max = 4194304

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20250819174030.1986278-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>