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 <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Signed-off-by: Chris Metcalf <[e-mail]>

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

···
 	return (val + c->high_bits) & ~rhs;
 }
 
+static inline unsigned long zero_bytemask(unsigned long mask)
+{
+	return ~1ul << __fls(mask);
+}
+
 #else
 
 #ifdef CONFIG_64BIT