Modernize geom_stats_snapshot_get

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.

Sponsored by:	Axcient
Reviewed by:	phk
Differential Revision:	https://reviews.freebsd.org/D29047

(cherry picked from commit f05b724ecb310fb91da1947ae6c68647f58f5f12)