[$] Modernizing swapping: the end of the swap map

The first installment in this series
introduced several data structures in the kernel’s swap subsystem and
described work to replace some of those with a new “swap table” structure.
The work did not stop there, though; there is more modernization of the
swap subsystem queued for an upcoming development cycle, and even more for
multiple kernel releases after that. Once that work is done, the swap
subsystem will be both simpler and faster than it is now.