ext2fs: Bring back the htree dir_index implementation.

The htree dir_index is perhaps one of the most characteristic
features of the linux ext3 implementation. It was removed
in r281670, due to repeated bug reports.

Damjan Jovanic detected and fixed three bugs and did some
stress testing by building Apache OpenOffice on top of it
so it is now in good shape to bring back.

Differential Revision: https://reviews.freebsd.org/D5007

Submitted by: Damjan Jovanovic
Reviewed by: pfg
Tested by: pho
Relnotes: Yes
MFC after: 2 months (only 10.x)