From Hugh.  Remove some strangeness in the swapoff path.

"it dates from the days when that block set *swap_map to 0, but in
 2.4.13 I changed it to set *swap_map to 1 and delete_from_swap_cache
 afterwards: it's been wrong ever since, hasn't it?  swap_list_locking
 can go, it was there to guard nr_swap_pages for si_swapinfo; the
 swap_device_locking has to stay because swap_map is an array of
 unsigned _shorts_."