From: Hugh Dickins <hugh@veritas.com>

Good example of "swapper_space considered harmful": swap_unplug_io_fn was
originally designed for calling via swapper_space.backing_dev_info; but
that way it loses track of which device is to be unplugged, so had to
unplug all swap devices.  But now sync_page tests SwapCache anyway, can
call swap_unplug_io_fn with page, which leads direct to the device.

Reverted -mc4's CONFIG_SWAP=n fix, just add another NOTHING for it.
Reverted -mc3's editorial adjustments to swap_backing_dev_info and
swapper_space initializations: they document the few fields which are
actually used now, as comment above them says (sound of slapped wrist).