sh64: failing __get_user() should zero

It could be done in exception-handling bits in __get_user_b() et.al.,
but the surgery involved would take more knowledge of sh64 details
than I have or _want_ to have.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/sh/include/asm/uaccess_64.h

@@ -24,6 +24,7 @@
 #define __get_user_size(x,ptr,size,retval)			\
 do {								\
 	retval = 0;						\
+	x = 0;							\
 	switch (size) {						\
 	case 1:							\
 		retval = __get_user_asm_b((void *)&x,		\