kernel - more kmalloc and nlookup performance optimizations

kernel - more kmalloc and nlookup performance optimizations

* Give the pcpu counters in struct malloc_type their own cache line per
  cpu.  This removes a large kmalloc/kfree bottleneck on multi-socket
  systems

* Avoid having to ref, lock, and GETATTR intermediate directory components
  in nlookup() by adding the NCF_WXOK flag.  This flag is set in the ncp
  when the directory permissions are at least 555.  This saves significant
  overhead in all situations, including single-threaded.

Discussed-with: Mateusz Guzik (mjg_)