netlink: Drop sock parameter from nl_msg_put_(ge)nlmsghdr().

netlink: Drop sock parameter from nl_msg_put_(ge)nlmsghdr().

These two functions use their "sock" parameter only to figure out the
nlmsg_pid to put in the nlmsghdr.  But that field can be filled in just
as well right before sending the message.  Since our functions for sending
Netlink messages always modify the nlmsghdr anyhow (to fill in the length),
there is little benefit to filling in the nlmsg_pid in advance.  The cost,
on the other hand, is having to pass another argument to functions that
already have too many.  So this commit removes the argument.