selftests: forwarding: Use sysctl_set(), sysctl_restore()

Instead of hand-managing the sysctl set and restore, use the wrappers
sysctl_set() and sysctl_restore() to do the bookkeeping automatically.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>