[IPV4]: OOPS with NETLINK_FIB_LOOKUP netlink socket

[ Regression added by changeset:
	cd40b7d3
	[NET]: make netlink user -> kernel interface synchronious
  -DaveM ]

nl_fib_input re-reuses incoming skb to send the reply. This means that this
packet will be freed twice, namely in:
- netlink_unicast_kernel
- on receive path
Use clone to send as a cure, the caller is responsible for kfree_skb on error.

Thanks to Alexey Dobryan, who originally found the problem.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>