atm: clip: Fix potential null-ptr-deref in to_atmarpd().

In the Linux kernel, the following vulnerability has been resolved:

atm: clip: Fix potential null-ptr-deref in to_atmarpd().

atmarpd is protected by RTNL since commit f3a0592b37b8 ("[ATM]: clip causes unregister hang").

However, it is not enough because to_atmarpd() is called without RTNL, especially clip_neigh_solicit() / neigh_ops->solicit() is unsleepable.

Also, there is no RTNL dependency around atmarpd.

Let's use a private mutex and RCU to protect access to atmarpd in to_atmarpd().