s390/qeth: restructure IP notification handlers

Extract a helper that does the actual work & returns the right NOTIFY_*
responses, and start putting the temporary ipaddr container objects
on the stack rather than kmalloc'ing them. They are small, and this
reduces the confusion of which objects actually get added to qeth's
IP tables.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>