rtnetlink: Protect link_ops by mutex.

rtnl_link_unregister() holds RTNL and calls synchronize_srcu(),
but rtnl_newlink() will acquire SRCU frist and then RTNL.

Then, we need to unlink ops and call synchronize_srcu() outside
of RTNL to avoid the deadlock.

   rtnl_link_unregister()       rtnl_newlink()
   ----                         ----
   lock(rtnl_mutex);
                                lock(&ops->srcu);
                                lock(rtnl_mutex);
   sync(&ops->srcu);

Let's move as such and add a mutex to protect link_ops.

Now, link_ops is protected by its dedicated mutex and
rtnl_link_register() no longer needs to hold RTNL.

While at it, we move the initialisation of ops->dellink and
ops->srcu out of the mutex scope.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241108004823.29419-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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