selftests: forwarding: lib: Split sysctl_save() out of sysctl_set()

In order to be able to save the current value of a sysctl without changing
it, split the relevant bit out of sysctl_set() into a new helper.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-5-petrm@nvidia.com


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