swap_ops updates v2

Hi all,

this series is a follow on to the swap ops series now in mm-unstable.

The first patch reintroduces direct folio writes for synchronous
swap files, the other two remove the double indirect for file
system based swap.

Changes since v1:
 - include swap_ops.h in zswap.c
 - fix a commit message typo
 - add a commit log blurb to mention there are not functional changes in
   patch 2