kernel - per-thread fd cache, p_fd lock bypass

kernel - per-thread fd cache, p_fd lock bypass

* Implement a per-thread (fd,fp) cache.  Cache hits can keep fp's
  in a held state (avoiding the need to fhold()/fdrop() the ref count),
  and bypasses the p_fd spinlock.  This allows the file pointer structure
  to generally be shared across cpu caches.

* Can cache up to four descriptors in each thread, LRU.  This is the common
  case.  Highly threaded programs tend to focus work on a distinct
  file descriptors in each thread.

* One file descriptor can be cached in up to four threads.  This is
  a significant limitation, though relatively uncommon.  On a cache miss
  the code drops into the normal shared p_fd spinlock lookup.