netlink: store user-provided rtm_protocol

Store user-supplied source protocol in the nexthops and nexthop groups.
Protocol specification help routing daemons like bird to quickly
identify self-originated routes after the crash or restart.

Example:

10.2.0.0/24 via 10.0.0.2 dev vtnet0 proto bird
10.3.0.0/24 proto bird
	nexthop via 10.0.0.2 dev vtnet0 weight 3
	nexthop via 10.0.0.3 dev vtnet0 weight 4