sysctl: Fix data races in proc_douintvec_minmax().

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

sysctl: Fix data races in proc_douintvec_minmax().

A sysctl variable 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.

This patch changes proc_douintvec_minmax() to use READ_ONCE() and

WRITE_ONCE() internally to fix data-races on the sysctl side. For now,

proc_douintvec_minmax() itself is tolerant to a data-race, but we still

need to add annotations on the other subsystem's side.