rtnetlink: Protect struct rtnl_link_ops with SRCU.

Once RTNL is replaced with rtnl_net_lock(), we need a mechanism to
guarantee that rtnl_link_ops is alive during inflight RTM_NEWLINK
even when its module is being unloaded.

Let's use SRCU to protect ops.

rtnl_link_ops_get() now iterates link_ops under RCU and returns
SRCU-protected ops pointer.  The caller must call rtnl_link_ops_put()
to release the pointer after the use.

Also, __rtnl_link_unregister() unlinks the ops first and calls
synchronize_srcu() to wait for inflight RTM_NEWLINK requests to
complete.

Note that link_ops needs to be protected by its dedicated lock
when RTNL is removed.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>