commit 2651cc69 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>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>