ng_l2tp_seq_reset() clears sequence number state for a node but does so
without holding the associated mutex. It stops callouts first but
without the mutex this is racy. This can be triggered by node shutdown
or a NGM_L2TP_SET_CONFIG message with "enabled" set to 0.
Also modify the shutdown code to block until callouts are finished
executing before freeing memory. ng_uncallout() does not, and cannot,
cancel a running callout.