futex: Fix spurious wakeup for requeue_pi really

The requeue_pi path doesn't use unqueue_me() (and the racy lock_ptr ==
NULL test) nor does it use the wake_list of futex_wake() which where
the reason for commit 41890f2 (futex: Handle spurious wake up)

See debugging discussing on LKML Message-ID: <[email protected]>

The changes in this fix to the wait_requeue_pi path were considered to
be a likely unecessary, but harmless safety net. But it turns out that
due to the fact that for unknown $@#!*( reasons EWOULDBLOCK is defined
as EAGAIN we built an endless loop in the code path which returns
correctly EWOULDBLOCK.

Spurious wakeups in wait_requeue_pi code path are unlikely so we do
the easy solution and return EWOULDBLOCK^WEAGAIN to user space and let
it deal with the spurious wakeup.

Cc: Darren Hart <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Dinakar Guniguntala <[email protected]>
LKML-Reference: <[email protected]>
Cc: [email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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