xtensa: re-wire umount syscall to sys_oldumount

commit 2651cc6974d47fc43bef1cd8cd26966e4f5ba306 upstream.

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.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>