Pull up following revision(s) (requested by knakahara in ticket #341):
sys/net/if_l2tp.c: revision 1.12
fix l2tp panic when l2tp session id is changed (same as if_vlan.c:r1.104)
E.g. the following operation causes this panic.
    ====================
    # ifconfig l2tp0 create
    # ifconfig l2tp0 session 140 140
    # ifconfig l2tp1 create
    # ifconfig l2tp1 session 200 200
    # ifconfig l2tp1 session 300 300
    panic: kernel diagnostic assertion "new->ple_next == NULL" failed: file "/disk4/home/k-nakahara/repos/netbsd-src/sys/sys/pslist.h", line 118
    ====================
Pointed out by s-yamaguchi@IIJ, thanks.

(snj)