l2tp: add session reorder queue purge function to core

If an l2tp session is deleted, it is necessary to delete skbs in-flight
on the session's reorder queue before taking it down.

Rather than having each pseudowire implementation reaching into the
l2tp_session struct to handle this itself, provide a function in l2tp_core to
purge the session queue.

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