selftests: futex: Add futex wait test

There are three different strategies to uniquely identify a futex in the
kernel:

 - Private futexes: uses the pointer to mm_struct and the page address

 - Shared futexes: checks if the page containing the address is a PageAnon:
   - If it is, uses the same data as a private futexes
   - If it isn't, uses an inode sequence number from struct inode and
      the page's index

Create a selftest to check those three paths and basic wait/wake
mechanism.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20210531165036.41468-2-andrealmeid@collabora.com