This code dates back to the very first diskless support on FreeBSD,
back when swapon(8) couldn't simply be run on a NFS backed file.
Suggested replacement command sequence on the client:
dd if=/dev/zero of=/swapfile bs=1k count=1 oseek=100000
swapon /swapfile
rm -f /swapfile
For whatever value of 100000 you want.