netlink: add a start callback for starting a netlink dump

netlink: add a start callback for starting a netlink dump

The start callback allows the caller to set up a context for the
dump callbacks. Presumably, the context can then be destroyed in
the done callback.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit fc9e50f5a5a4e1fa9ba2756f745a13e693cf6a06)

Orabug: 27169581
CVE: CVE-2017-16939

Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>