arch/powerpc: provide zero_bytemask() for big-endian

For some reason, only the little-endian flavor of
powerpc provided the zero_bytemask() implementation.

Reported-by: Michal Sojka <sojkam1@fel.cvut.cz>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
diff --git a/arch/powerpc/include/asm/word-at-a-time.h b/arch/powerpc/include/asm/word-at-a-time.h
index 5b3a903..e4396a7 100644
--- a/arch/powerpc/include/asm/word-at-a-time.h
+++ b/arch/powerpc/include/asm/word-at-a-time.h
@@ -40,6 +40,11 @@
 	return (val + c->high_bits) & ~rhs;
 }
 
+static inline unsigned long zero_bytemask(unsigned long mask)
+{
+	return ~1ul << __fls(mask);
+}
+
 #else
 
 #ifdef CONFIG_64BIT