puffs - don't lock dvp on nresolve

puffs - don't lock dvp on nresolve

 * locking dvp on nresolve causes a deadlock with the puffsop thread
   when the dvp is the root vnode.

 * If a lookup/nresolve occurs at the same time as a flush via the
   puffsop, then puffsop will not be able to acquire a lock on the root
   vnode as the lookup routine already holds it. Similarly the lookup
   routine never gets woken up waiting for the message to arrive, since
   puffsop is stuck and can't reply.