l2tp: use IDR for all session lookups

Add generic session getter which uses IDR. Replace all users of
l2tp_tunnel_get_session which uses the per-tunnel session list to use
the generic getter.

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