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

lib/libgeom/geom_stats.c

@@ -136,9 +136,8 @@ geom_stats_snapshot_get(void)
 		free(sp);
 		return (NULL);
 	}
-	memset(sp->ptr, 0, pagesize * npages); 	/* page in, cache */
+	explicit_bzero(sp->ptr, pagesize * npages); 	/* page in, cache */
 	clock_gettime(CLOCK_REALTIME, &sp->time);
-	memset(sp->ptr, 0, pagesize * npages); 	/* page in, cache */
 	memcpy(sp->ptr, statp, pagesize * npages);
 	sp->pages = npages;
 	sp->perpage = spp;