rtnetlink: Call rtnl_link_get_net_capable() in do_setlink().

We will push RTNL down to rtnl_setlink().

RTM_SETLINK could call rtnl_link_get_net_capable() in do_setlink()
to move a dev to a new netns, but the netns needs to be fetched before
holding rtnl_net_lock().

Let's move it to rtnl_setlink() and pass the netns to do_setlink().

Now, RTM_NEWLINK paths (rtnl_changelink() and rtnl_group_changelink())
can pass the prefetched netns to do_setlink().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>