cxgb4i: avoid holding mutex in interrupt context

cxgbi_inet6addr_handler() can be called in interrupt context, so use rcu
protected list while finding netdev

Applies on top of core-for-3.18

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Karen Xie <kxie@chelsio.com>
Fixes: fc8d0590d914 ("libcxgbi: Add ipv6 api to driver")
Fixes: 759a0cc5a3e1 ("cxgb4i: Add ipv6 code to driver, call into libcxgbi ipv6
api")
Signed-off-by: Christoph Hellwig <hch@lst.de>