ipv6: addrconf: don't use rtnl mutex in RTM_GETNETCONF

Instead of relying on rtnl mutex bump device reference count.
After this change, values reported can change in parallel, but thats not
much different from current state, as anyone can change the settings
right after rtnl_unlock (and before userspace processed reply).

While at it, switch to GFP_KERNEL allocation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>