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To keep CLs small on userspace swap this CL only adds the configuration
tunables which will be available to userspace swap.

Additionally, it adds a helper to determine if the kernel supports the
necessary features for userspace swap.

BUG=chromium:1067833

Change-Id: Id674abede3d503218189cd3b4bef130c4297b5c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288492
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786468}