futex: Remove {get,drop}_futex_key_refs()

Now that {get,drop}_futex_key_refs() have become a glorified NOP,
remove them entirely.

The only thing get_futex_key_refs() is still doing is an smp_mb(), and
now that we don't need to (ab)use existing atomic ops to obtain them,
we can place it explicitly where we need it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>