xfrm: take care of request sockets

TCP SYNACK messages might now be attached to request sockets.

XFRM needs to get back to a listener socket.

Adds new helpers that might be used elsewhere :
sk_to_full_sk() and sk_const_to_full_sk()

Note: We also need to add RCU protection for xfrm lookups,
now TCP/DCCP have lockless listener processing. This will
be addressed in separate patches.

Fixes: ca6fb065 ("tcp: attach SYNACK messages to request sockets instead of listener")
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>