l2tp: fix unused variable warning

net/l2tp/l2tp_core.c:1111:15: warning: unused variable
'sk' [-Wunused-variable]

Fixes: 31c70d59 ("l2tp: keep original skb ownership")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>