ip: Fix data-races around sysctl_ip_prot_sock.
In the Linux kernel, the following vulnerability has been resolved:

ip: Fix data-races around sysctl_ip_prot_sock.

sysctl_ip_prot_sock is accessed concurrently, and there is always a chance
of data-race.  So, all readers and writers need some basic protection to
avoid load/store-tearing.