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

Vulnerability Description

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().

References

• https://git.kernel.org/stable/c/a4c5785feb979cd996a99cfaad8bf353b2e79301
• https://git.kernel.org/stable/c/70eac9ba7ce25d99c1d99bbf4ddb058940f631f9
• https://git.kernel.org/stable/c/3251ce3979f41bd228f77a7615f9dd616d06a110
• https://git.kernel.org/stable/c/ee4d9e4ddf3f9c4ee2ec0a3aad6196ee36d30e57
• https://git.kernel.org/stable/c/06935c50cfa3ac57cce80bba67b6d38ec1406e92
• https://git.kernel.org/stable/c/36caab990b69ef4eec1d81c52a19f080b7daa059
• https://git.kernel.org/stable/c/f58e4270c73e7f086322978d585ea67c8076ce49
• https://git.kernel.org/stable/c/706cc36477139c1616a9b2b96610a8bb520b7119