netlink: mmaped netlink: ring setup

Add support for mmap'ed RX and TX ring setup and teardown based on the
af_packet.c code. The following patches will use this to add the real
mmap'ed receive and transmit functionality.

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

net/Kconfig

@@ -23,6 +23,15 @@ menuconfig NET
 
 if NET
 
+config NETLINK_MMAP
+	bool "Netlink: mmaped IO"
+	help
+	  This option enables support for memory mapped netlink IO. This
+	  reduces overhead by avoiding copying data between kernel- and
+	  userspace.
+
+	  If unsure, say N.
+
 config WANT_COMPAT_NETLINK_MESSAGES
 	bool
 	help