parisc: fix ipv6 checksum

ipv6 recently started exhibiting the same symptoms as ipv4 was, add
a memory clobber around inline checksum assembly that fribbles memory
to ensure gcc doesn't erroneously cache across it.

Signed-off-by: Kyle McMartin <[e-mail]>

arch/parisc/include/asm/checksum.h

···
 #endif
 	: "=r" (sum), "=r" (saddr), "=r" (daddr), "=r" (len)
 	: "0" (sum), "1" (saddr), "2" (daddr), "3" (len), "r" (proto)
-	: "r19", "r20", "r21", "r22");
+	: "r19", "r20", "r21", "r22", "memory");
 	return csum_fold(sum);
 }