It's not really useful in a jail or in a mdroot or even if a users
wants to do a full zfs machine.
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D36227
Specifically reading is done if ffs_sbget() and writing is done
in ffs_sbput(). These functions are exported to libufs via the
sbget() and sbput() functions which then used in the various
filesystem utilities. This work is in preparation for adding
subperblock check hashes.
No functional change intended.
Reviewed by: kib