VFS accounting: add in-memory storage counters

VFS accounting: add in-memory storage counters

* Use a red-black tree of small arrays to store uid and gid accounting
  information.
  User and group ids often come in small groups of consecutive numbers;
  small arrays have a high probability of having more than one element
  used at the same time, reducing pointer chasing in the binary tree.

* Also use a global per mount-point counter

* Only enable data collection for volumes which have been properly
  initialized

* Protect the counters by a per mount-point spinlock