futex: teach about shared vs private

Linux 3.18 will include support for stressing both shared and
private futexes (we currently only support this for hashing).
For now, just print the type of futex in the report, when updating
the futexbench version in mmtests, we will then add runs with the
-S option.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>