There are a couple of statistical functions which scan the entire swap
map counting things up, to display in /proc.

On my machine, these hold spinlocks for 19 milliseconds which is
unacceptable from a scheduling latency point of view.

And an application which sits in a loop reading /proc/swaps on a large
machine is probably a decent denial-of-service attack - it will limit
swap allocations to tens of pages per second.

So add a counter to swap_info_struct and use it to track how many pages
are currently in use, so those reporting functions don't need to add
them all up.