fork: Copy the vm_stacktop field into the new vmspace

Fixes: 1811c1e957ee ("exec: Reimplement stack address randomization")
Reported by: pho
Reported by: syzbot+0446312a51bc13ead834@syzkaller.appspotmail.com
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 46d35d415aa9664b0ddc98a76e453eec20af0016)