cifs: fix return value in cifsConvertToUTF16

This function returns the wrong value, which causes the callers to get
the length of the resulting pathname wrong when it contains non-ASCII
characters.

This seems to fix https://bugzilla.samba.org/show_bug.cgi?id=6767

Cc: <stable@vger.kernel.org>
Reported-by: Baldvin Kovacs <baldvin.kovacs@gmail.com>
Reported-and-Tested-by: Nicolas Lefebvre <nico.lefebvre@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>