vrf: add sysctl parameter for strict mode

Add net.vrf.strict_mode sysctl parameter.

When net.vrf.strict_mode=0 (default) it is possible to associate multiple
VRF devices to the same table. Conversely, when net.vrf.strict_mode=1 a
table can be associated to a single VRF device.

When switching from net.vrf.strict_mode=0 to net.vrf.strict_mode=1, a check
is performed to verify that all tables have at most one VRF associated,
otherwise the switch is not allowed.

The net.vrf.strict_mode parameter is per network namespace.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: David S. Miller <davem@davemloft.net>