Rework locking, that I have introduced recently, since it was incorrect:

Rework locking, that I have introduced recently, since it was incorrect:

  First, mutexed callouts are incompatible with netgraph nodes, because
  netgraph(4) can guarantee that the function will be called with mutex
  held.

  Second, nodes should not send data to their neighbor holding their
  mutex. A node does not know what stack can it enter sending data in
  some direction. May be executing will encounter a place to sleep.

New locking:

  - ng_pptpgre_recv() and ng_pptpgre_xmit() must be entered with mutex held.
  - ng_pptpgre_recv() and ng_pptpgre_xmit() unlock mutex before
    sending data and then return unlocked.
  - callout routines acquire mutex themselves.