MFC r310705, r310706:
style(9) cleanups.

Just to reduce some of the issues found with indent(1).

MFC r294504, r294652, r294653, r294655:

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
RelNotes: yes

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

MFC r277301:
ext2: cosmetical issues

Minor sorting and note when the cases are expected to fall through.

MFC r258904, r259780:
Small ext2fs updates.

Add two new reserved inodes.
Make the hashing algorithm match the linux code.

PR: kern/183230