The FreeBSD fstyp supports HAMMER1/2 based on somewhat old DragonFly source.
Below diff is to make them uptodate with the current DragonFly's fstyp HAMMER1/2 code base without making changes to other fstyp files (note that DragonFly does this in their fstyp).
This makes easier to sync up with DragonFly's HAMMER1/2 detection support in the future.
Git format diff
Looks good to me, thank you!
FWIW, I am considering dropping the headers and using only the required structures directly.
A commit references this bug:
Date: Thu Feb 6 18:04:16 UTC 2020
New revision: 357631
fstyp: sync HAMMER1/2 detection support with DragonFly BSD.
Submitted by: Tomohiro Kusumi