unix: use vm.swap_total for FreeBSD TestSysctlUint64

Fixes golang/go#21375

Change-Id: Ibb33df9d71eb3a935a1f8f9f0ad1f180211e8ce4
Reviewed-on: https://go-review.googlesource.com/54390
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed