In order to be able to support proper RST messages for TCP MD5 flows, we
need to allow access to MD5 keys without locking listener socket.

This conversion is a nice cleanup, and shrinks size of timewait sockets
by 80 bytes.

IPv6 code reuses generic code found in IPv4 instead of duplicating it.

Control path uses GFP_KERNEL allocations instead of GFP_ATOMIC.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Cc: Shawn Lu <shawn.lu@ericsson.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>