netlink: Expose method to get Netlink pid of a socket.

netlink: Expose method to get Netlink pid of a socket.

In the future, the kernel will use unicast messages instead of
multicast to send upcalls.  As a result, we need to be able to
tell it where to direct the traffic.  This adds a function to expose
the Netlink pid of a socket so it can be included in messages to the
kernel.