ng_l2tp: improve seq structure locking.

Cover few cases of access to seq without lock missed in 702f98951d5.
There are no known bugs fixed with this change, however. With INVARIANTS
embed ng_l2tp_seq_check() into lock/unlock macros. Slightly reduce number
of locks/unlocks per packet keeping the lock between functions.

Reviewed by: mjg, markj
Differential Revision: https://reviews.freebsd.org/D31476