s390/qeth: add wrapper for IP table access

Extract a little helper, so that high-level callers can manipulate the
IP table without worrying about the locking. This will make it easier
to convert the code to a different locking primitive later on.

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