Lockless write to seq->inproc in M_PREPEND failure path: data race

Summary

ng_l2tp_rcvdata_lower(:957-967): seq->inproc=1 under mtx_lock(:957) then unlock(:958). M_PREPEND(:961) on failure clears seq->inproc=0(:963) WITHOUT holding mtx. All other inproc access (:942 read, :975 write) under mutex. Data race -> transient control-packet pipeline inconsistency (unnecessary ZLB drop or premature 2nd packet forward). Self-correcting. No memory corruption.