Add kernel implementation of getcwd() which uses this cache, falling
back to reading the filesystem on a cache miss.
Along for the ride: add new VOP_FSYNC flag FSYNC_RECLAIM indicating
that a reclaim is being done, so only a "shallow" fsync is needed.
The only benefit this provides is that we don't use kmem_map to map the memory
used for name cache entries (though, this is a 13 virtual page savings on my
PPro) since vnodes are never freed (they have their own freelist).
suggested fix was correct: the suggested change to cache_lookup would cause
the counters to be incremented when doingcache was zero, and the suggested
change to cache_enter was prone to a race condition (e.g. if doingcache
became 1 between the cache_lookup and cache_enter).
sommerfe