um: Fix waitpid() usage in helper code

If UML is executing a helper program it is using
waitpid() with the __WCLONE flag to wait for the program
as the helper is executed from a clone()'ed thread.
While using __WCLONE is perfectly fine for clone()'ed
childs it won't detect terminated childs if the helper
has issued an execve().

We have to use __WALL to wait for both clone()'ed and
regular childs to detect the termination before and
after an execve().

Reported-and-tested-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Richard Weinberger <richard@nod.at>