use rb-tree for directory lookups

use rb-tree for directory lookups

* tmpfs directories are structured as lists of directory entries; this
  leads to linear lookup costs. Directories with many files become fairly
  expensive to operate on.

* This patch uses a rb-tree keyed on the name of the searched file to
  speed up lookups

* Besides this rb-tree implementation, a hash version was also tested.
  Both gave solid performance enhancements compared to the previous tmpfs
  implementation.
  The rb-tree version was the faster of the two and thus becomes the
  choosen one.

* See issue 2375 for details and performance numbers
  https://bugs.dragonflybsd.org/issues/2375