dup2: Fix return value with oldfd == newfd and invalid fd

The return value of dup2 when oldfd == newfd and the fd isn't valid is
not getting properly sign extended.  We end up with 4294967287 instead
of -EBADF.

I've reproduced this on SLE11 (2.6.27.21), openSUSE Factory
(2.6.29-rc5), and Ubuntu 9.04 (2.6.28).

This patch uses a signed int for the error value so it is properly
extended.

Commit 6c5d0512 introduced this
regression.
Reported-by: NJiri Dluhos <jdluhos@novell.com>
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/fcntl.c

@@ -117,11 +117,13 @@ SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
 {
 	if (unlikely(newfd == oldfd)) { /* corner case */
 		struct files_struct *files = current->files;
+		int retval = oldfd;
+
 		rcu_read_lock();
 		if (!fcheck_files(files, oldfd))
-			oldfd = -EBADF;
+			retval = -EBADF;
 		rcu_read_unlock();
-		return oldfd;
+		return retval;
 	}
 	return sys_dup3(oldfd, newfd, 0);
 }