x86-64: Fix the failure case in copy_user_handle_tail()

commit 66db3feb upstream.

The increment of "to" in copy_user_handle_tail() will have incremented
before a failure has been noted.  This causes us to skip a byte in the
failure case.

Only do the increment when assured there is no failure.
Signed-off-by: CQ Tang <cq.tang@intel.com>
Link: http://lkml.kernel.org/r/20130318150221.8439.993.stgit@phlsvslse11.ph.intel.comSigned-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/lib/usercopy_64.c

@@ -169,10 +169,10 @@ copy_user_handle_tail(char *to, char *from, unsigned len, unsigned zerorest)
 	char c;
 	unsigned zero_len;
 
-	for (; len; --len) {
+	for (; len; --len, to++) {
 		if (__get_user_nocheck(c, from++, sizeof(char)))
 			break;
-		if (__put_user_nocheck(c, to++, sizeof(char)))
+		if (__put_user_nocheck(c, to, sizeof(char)))
 			break;
 	}