s390: avoid inconsistent lock state in qeth

ipv6_regen_rndid in net/ipv6/addrconf.c makes use of "write_lock_bh"
for its inet6_dev->lock. It may run in softirq-context.
qeth makes use of "read_lock" for the same inet6_dev->lock.
To avoid a potential deadlock situation, qeth should make use of
"read_lock_bh" for its usages of inet6_dev->lock.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>