l2tp: add session reorder queue purge function to core

l2tp: add session reorder queue purge function to core

commit 48f72f92b31431c40279b0fba6c5588e07e67d95 upstream.

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>
[bwh: Backported to 3.2: use non-atomic increment on rx_errors]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>