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