netlink: add reference of module in netlink_dump_start

netlink: add reference of module in netlink_dump_start

I get a panic when I use ss -a and rmmod inet_diag at the
same time.

It's because netlink_dump uses inet_diag_dump which belongs to module
inet_diag.

I search the codes and find many modules have the same problem.  We
need to add a reference to the module which the cb->dump belongs to.

Thanks for all help from Stephen,Jan,Eric,Steffen and Pablo.

Change From v3:
change netlink_dump_start to inline,suggestion from Pablo and
Eric.

Change From v2:
delete netlink_dump_done,and call module_put in netlink_dump
and netlink_sock_destruct.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>