genetlink: check for callback type at op load time

Now that genl_get_cmd_split() is informed what type of callback
user is trying to access (do or dump) we can check that this
callback is indeed available and return an error early.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>