net: add sk_fullsock() helper

We have many places where we want to check if a socket is
not a timewait or request socket. Use a helper to avoid
hard coding this.

[Backport of net-next 1d0ab253]
[lorenzo@google.com: removed TCPF_NEW_SYN_RECV, and added a comment to add it back.]

Bug: 24163529
Change-Id: Ibf09017e1ab00af5e6925273117c335d7f515d73
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>