[S390] Hipersocket multicast queue: make sure outbound handler is called

A HiperSocket multicast queue works asynchronously. When sending
buffers, the buffer state change from PRIMED to EMPTY may happen
delayed. Reschedule the checking for changes in the outbound queue,
if there are still PRIMED buffers.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>