selftests: net: tests net.core.{r,w}mem_{default,max} sysctls in a netns

Add a selftest which checks that the sysctl is present in a netns,
that the value is read from the init one, and that it's readonly.

Signed-off-by: Matteo Croce <teknoraver@meta.com>
Link: https://lore.kernel.org/r/20240530232722.45255-3-technoboy85@gmail.com


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