l2tp: have l2tp_ip_destroy_sock use ip_flush_pending_frames

Use the recently exported ip_flush_pending_frames instead of a
free-coded version and lock the socket while we call it.

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