selftests/net: optmem_max became per netns

/proc/sys/net/core/optmem_max is now per netns, change two tests
that were saving/changing/restoring its value on the parent netns.

Change-Id: Idacd74a41133cbb849cca0d608a02dcad97da92b
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 18872ba8cd2406ffa835f9f6276e47ed86fbb5d6)
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/62584
Tested-by: Mina Almasry <almasrymina@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/63177
Reviewed-by: Mina Almasry <almasrymina@google.com>
Tested-by: Minh-Anh Nguyen <minhanhdn@google.com>