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: NMichal Sojka <sojkam1@fel.cvut.cz>
Acked-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NChris Metcalf <cmetcalf@ezchip.com>

arch/powerpc/include/asm/word-at-a-time.h

@@ -40,6 +40,11 @@ static inline bool has_zero(unsigned long val, unsigned long *data, const struct
 	return (val + c->high_bits) & ~rhs;
 }
 
+static inline unsigned long zero_bytemask(unsigned long mask)
+{
+	return ~1ul << __fls(mask);
+}
+
 #else
 
 #ifdef CONFIG_64BIT