x86/xen/msr: Fix uninitialized variable 'err'

xen_read_msr_safe() currently passes an uninitialized argument 'err' to
xen_do_read_msr(). But as xen_do_read_msr() may not set the argument,
xen_read_msr_safe() could return err with an unpredictable value.

To ensure correctness, initialize err to 0 (representing success)
in xen_read_msr_safe().

Do the same in xen_read_msr(), even err is not used after being passed
to xen_do_read_msr().

Closes: https://lore.kernel.org/xen-devel/[email protected]/
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Xin Li (Intel) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: xinli-intel

arch/x86/xen/enlighten_pv.c

@@ -1162,7 +1162,7 @@ static void xen_do_write_msr(u32 msr, u64 val, int *err)
 
 static int xen_read_msr_safe(u32 msr, u64 *val)
 {
-	int err;
+	int err = 0;
 
 	*val = xen_do_read_msr(msr, &err);
 	return err;
@@ -1179,7 +1179,7 @@ static int xen_write_msr_safe(u32 msr, u64 val)
 
 static u64 xen_read_msr(u32 msr)
 {
-	int err;
+	int err = 0;
 
 	return xen_do_read_msr(msr, xen_msr_safe ? &err : NULL);
 }