[PowerPC] Fix atomic_cmpset_masked().

A recent kernel change caused the previously unused atomic_cmpset_masked() to
be used.

It had a typo in it.

Instead of reading the old value from an uninitialized variable, read it
from the passed-in pointer as intended.

This fixes crashes on 64 bit Book-E.

Obtained from: jhibbits

(cherry picked from commit 9941cb0657d63b4b1570dcf384196bb599bf9024)