powerpc: Fix smp_mb__before_spinlock()

Currently, smp_mb__before_spinlock() is defined to be smp_wmb()
in core code, but this is not sufficient on PowerPC.  This patch
therefore supplies an override for the generic definition to
strengthen smp_mb__before_spinlock() to smp_mb(), as is needed
on PowerPC.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <linuxppc-dev@lists.ozlabs.org>

arch/powerpc/include/asm/barrier.h

@@ -89,5 +89,6 @@ do {									\
 
 #define smp_mb__before_atomic()     smp_mb()
 #define smp_mb__after_atomic()      smp_mb()
+#define smp_mb__before_spinlock()   smp_mb()
 
 #endif /* _ASM_POWERPC_BARRIER_H */