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: Jiri Dluhos <jdluhos@novell.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>