netlink: implement memory mapped sendmsg()

Add support for mmap'ed sendmsg() to netlink. Since the kernel validates
received messages before processing them, the code makes sure userspace
can't modify the message contents after invoking sendmsg(). To do that
only a single mapping of the TX ring is allowed to exist and the socket
must not be shared. If either of these two conditions does not hold, it
falls back to copying.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>