net: genetlink: push doit/dumpit code from genl_family_rcv_msg

Currently the function genl_family_rcv_msg() is quite big. Since it is
quite convenient, push code that is related to doit and dumpit ops into
separate functions.

Do small changes on the way, like rc/err unification, NULL check etc.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>