fix(mkfs): reserve correct htree checksum tail

[adam-rl]

What this PR does / why we need it:

Fixes corrupt ext4 directory indexes created when metadata_csum is enabled. The pinned e2fsprogs fork reserved a 12-byte directory-entry checksum tail in an HTREE root, but HTREE roots use an 8-byte ext2_dx_tail. On 4 KiB filesystems this wrote limit 506 where the kernel and e2fsck require 507.

Adds an E2E regression that converts a directory with 1,000 entries and requires e2fsck -fn to pass.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update? No.
• Does this introduce breaking changes that would require an announcement or bumping the major version? No.
• Do all new files have an appropriate license header? The new file is a patch to the existing LGPL source.