lt2p: Fix possible WARN_ON from socket code when UDP socket is closed

If an L2TP daemon closes a tunnel socket while packets are queued in
the tunnel's reorder queue, a kernel warning is logged because the
socket is closed while skbs are still referencing it. The fix is to
purge the queue in the socket's release handler.

WARNING: at include/net/sock.h:351 udp_lib_unhash+0x41/0x68()
Pid: 12998, comm: openl2tpd Not tainted 2.6.25 #8
 [<c0423c58>] warn_on_slowpath+0x41/0x51
 [<c05d33a7>] udp_lib_unhash+0x41/0x68
 [<c059424d>] sk_common_release+0x23/0x90
 [<c05d16be>] udp_lib_close+0x8/0xa
 [<c05d8684>] inet_release+0x42/0x48
 [<c0592599>] sock_release+0x14/0x60
 [<c059299f>] sock_close+0x29/0x30
 [<c046ef52>] __fput+0xad/0x15b
 [<c046f1d9>] fput+0x17/0x19
 [<c046c8c4>] filp_close+0x50/0x5a
 [<c046da06>] sys_close+0x69/0x9f
 [<c04048ce>] syscall_call+0x7/0xb

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