Enable dfs submounts to handle remote referrals.

Having remote dfs root support in cifs_mount, we can
afford to pass into it UNC that is remote.

Signed-off-by: Igor Mammedov <niallain@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>