MFC r281670, r281703:

MFC	r281670, r281703:
Drop experimental ext2fs dir_index support.

The htree directory index is a highly desirable feature for research
purposes and was meant to improve performance in our ext2/3 driver.
Unfortunately our implementation has two problems:

- It never really delivered any performance improvement.
- It appears to corrupt the filesystem in undetermined circumstances.

Strictly speaking dir_index is not required for read/write support in
ext2/3 and our limited ext4 support still works fine without it.

Regain stability in the ext2 driver by removing it. We may need it back
(fixed) if we want to support encrypted ext4 support but thanks to the
wonders of version control we can always revert this change and bring it
back.

PR:	191895
PR:	198731
PR:	199309

git-svn-id: svn://svn.freebsd.org/base/stable/9@281842 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f