l2tp: handle hash key collisions in l2tp_v3_session_get

To handle colliding l2tpv3 session IDs, l2tp_v3_session_get searches a
hashed list keyed by ID and sk. Although unlikely, if hash keys
collide, it is possible that hash_for_each_possible loops over a
session which doesn't have the ID that we are searching for. So check
for session ID match when looping over possible hash key matches.

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>