l2tp: avoid duplicated code in l2tp_tunnel_closeall

l2tp_tunnel_closeall is called as a part of tunnel shutdown in order to
close all the sessions held by the tunnel.  The code it uses to close a
session duplicates what l2tp_session_delete does.

Rather than duplicating the code, have l2tp_tunnel_closeall call
l2tp_session_delete instead.

This involves a very minor change to locking in l2tp_tunnel_closeall.
Previously, l2tp_tunnel_closeall checked the session "dead" flag while
holding tunnel->hlist_lock.  This allowed for the code to step to the
next session in the list without releasing the lock if the current
session happened to be in the process of closing already.

By calling l2tp_session_delete instead, l2tp_tunnel_closeall must now
drop and regain the hlist lock for each session in the tunnel list.
Given that the likelihood of a session being in the process of closing
when the tunnel is closed, it seems worth this very minor potential
loss of efficiency to avoid duplication of the session delete code.

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