selftest: Fix set of ping_group_range in fcnal-test

ping_group_range sysctl has a compound value which does not go
through the various function layers in tact. Create a helper
function to bypass the layers and correctly set the value.

Signed-off-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240124214117.24687-3-dsahern@kernel.org


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