Fix a race condition between nlookup and vnode reclamation. Even though the

Fix a race condition between nlookup and vnode reclamation.  Even though the
cache_lock() refs a vnode to prevent reclamation, it is possible for a vnode
to already be in the middle of a reclamation as of the time the lock is
obtained.  Adjust cache_get*() and cache_resolve*() to check for the
condition and de-resolve the ncp before returning.

Reported-by: joerg@britannica.bec.de