powerpc: Fix write-after-destroy in lock elision [BZ #20822]

The update of *adapt_count after the release of the lock causes a race
condition when thread A unlocks, thread B continues and destroys the
mutex, and thread A writes to *adapt_count.

(cherry picked from commit e9a96ea1)
(with changes from commit eb1321f2)

(cherry picked from commit 2762a714)