l2tp: fix oops in L2TP IP sockets for connect() AF_UNSPEC case

An application may call connect() to disconnect a socket using an
address with family AF_UNSPEC. The L2TP IP sockets were not handling
this case when the socket is not bound and an attempt to connect()
using AF_UNSPEC in such cases would result in an oops. This patch
addresses the problem by protecting the sk_prot->disconnect() call
against trying to unhash the socket before it is bound.

The L2TP IPv4 and IPv6 sockets have the same problem. Both are fixed
by this patch.

The patch also adds more checks that the sockaddr supplied to bind()
and connect() calls is valid.

 RIP: 0010:[<ffffffff82e133b0>]  [<ffffffff82e133b0>] inet_unhash+0x50/0xd0
 RSP: 0018:ffff88001989be28  EFLAGS: 00010293
 Stack:
  ffff8800407a8000 0000000000000000 ffff88001989be78 ffffffff82e3a249
  ffffffff82e3a050 ffff88001989bec8 ffff88001989be88 ffff8800407a8000
  0000000000000010 ffff88001989bec8 ffff88001989bea8 ffffffff82e42639
 Call Trace:
 [<ffffffff82e3a249>] udp_disconnect+0x1f9/0x290
 [<ffffffff82e42639>] inet_dgram_connect+0x29/0x80
 [<ffffffff82d012fc>] sys_connect+0x9c/0x100

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>