powerpc: correct register usage in 64-bit syscall exit path

Since we don't restore the volatile registers in the syscall exit
path, we need to make sure we don't leak any potentially interesting
values from the kernel to userspace.  This was already the case for
all except r11.  This makes it use r11 for an MSR value, so r11 will
have an (uninteresting) MSR value in it on return to userspace.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/entry_64.S

@@ -183,8 +183,8 @@ syscall_exit_trace_cont:
 	ld	r13,GPR13(r1)		/* returning to usermode */
 1:	ld	r2,GPR2(r1)
 	li	r12,MSR_RI
-	andc	r10,r10,r12
-	mtmsrd	r10,1			/* clear MSR.RI */
+	andc	r11,r10,r12
+	mtmsrd	r11,1			/* clear MSR.RI */
 	ld	r1,GPR1(r1)
 	mtlr	r4
 	mtcr	r5