[NETNS][DST]: Add the network namespace pointer in dst_ops

The network namespace pointer can be stored into the dst_ops structure.
This is usefull when there are multiple instances of the dst_ops for a
protocol. When there are no several instances, this field will be never
used in the protocol. So there is no impact for the protocols which do
implement the network namespaces.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>