Modernize geom_stats_snapshot_get

• A logically useless memset() is used to fault in some memory pages. Change it to explicit_bzero so the compiler won't eliminate it.

• Eliminate the second memset. It made sense in the days of the Big Kernel Lock, but not in the days of fine-grained SMP and especially not in the days of VDSO.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: phk
Differential Revision: https://reviews.freebsd.org/D29047