geom: zero: Let sysctls .byte and .clear to be settable in loader
There is no reason to not allow kern.geom.zero.byte and
kern.geom.zero.clear to be settable as a tunable.
Reviewed by: imp, markj
Approved by: markj (mentor)
MFC after: 1 week
Event: EuroBSDCon 2025
Differential Revision: https://reviews.freebsd.org/D52763
(cherry picked from commit ae17bca63f0a03e6e46ea679a5be7f8851f82099)