net: make net.core.{r,w}mem_{default,max} namespaced

The following sysctl are global and can't be read from a netns:

net.core.rmem_default
net.core.rmem_max
net.core.wmem_default
net.core.wmem_max

Make the following sysctl parameters available readonly from within a
network namespace, allowing a container to read them.

Signed-off-by: Matteo Croce <teknoraver@meta.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://lore.kernel.org/r/20240530232722.45255-2-technoboy85@gmail.com


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