I copied the logic from ll/sc arch implementations, but that
was wrong and makes no sense at all.  Just do a straight
compare-exchange instruction, just like x86.

Based upon bug reports from Dennis Gilmore and Fabio Massimo.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>