FROMLIST: selftests/futex: add futex_swap selftest

This is the final patch in FUTEX_SWAP patchset. It
adds a test/benchmark to validate behavior and
compare performance of a new FUTEX_SWAP futex operation.

Detailed API design and behavior considerations are provided
in the commit messages of the previous two patches.

Signed-off-by: Peter Oskolkov <posk@google.com>
(am from https://lore.kernel.org/patchwork/patch/1277906/)
(also found at https://lore.kernel.org/r/20200722234538.166697-4-posk@posk.io

)

BUG=b:170967073
TEST=eve-kernelnext and manual verification with selftests

Signed-off-by: Brian Geffon <bgeffon@chromium.org>
Change-Id: I2b5b87a7f7ba3d4a7f604fc2e127941822090fe6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2476742


Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Joel Fernandes <joelaf@google.com>