Most system calls taking flags first check that the flags passed in are
valid, and that helps userspace to detect when new flags are supported.
But swapon never did so: start checking now, to help if we ever want to
support more swap_flags in future.
It's difficult to get stray bits set in an int, and swapon is not widely
used, so this is most unlikely to break any userspace; but we can just
revert if it turns out to do so.