xtensa: re-wire umount syscall to sys_oldumount

Userspace actually passes single parameter (path name) to the umount
syscall, so new umount just fails. Fix it by requesting old umount
syscall implementation and re-wiring umount to it.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>