Fix "sysctl vm.vmtotal" output on machines with > 2TB virtual memory

sbin/sysctl/sysctl.c
Fix integer overflows in printf format strings

PR: 199673
Submitted by: Vitaly Magerya
Reviewed by: cem
Approved by: re (marius)
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6941