microblaze: Fix /dev/zero corruption from __clear_user()

A userland read of more than PAGE_SIZE bytes from /dev/zero results in
(a) not all of the bytes returned being zero, and
(b) memory corruption due to zeroing of bytes beyond the user buffer.

This is caused by improper constraints on the assembly __clear_user function.
The constrints don't indicate to the compiler that the pointer argument is
modified. Since the function is inline, this results in double-incrementing
of the pointer when __clear_user() is invoked through a multi-page read() of
/dev/zero.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Acked-by: Michal Simek <monstr@monstr.eu>
CC: stable@kernel.org

arch/microblaze/include/asm/uaccess.h

@@ -120,16 +120,16 @@ static inline unsigned long __must_check __clear_user(void __user *to,
 {
 	/* normal memset with two words to __ex_table */
 	__asm__ __volatile__ (				\
-			"1:	sb	r0, %2, r0;"	\
+			"1:	sb	r0, %1, r0;"	\
 			"	addik	%0, %0, -1;"	\
 			"	bneid	%0, 1b;"	\
-			"	addik	%2, %2, 1;"	\
+			"	addik	%1, %1, 1;"	\
 			"2:			"	\
 			__EX_TABLE_SECTION		\
 			".word	1b,2b;"			\
 			".previous;"			\
-		: "=r"(n)				\
-		: "0"(n), "r"(to)
+		: "=r"(n), "=r"(to)			\
+		: "0"(n), "1"(to)
 	);
 	return n;
 }