x86/cpu: Re-enable stackprotector

Commit 5416c266 ("x86: make sure load_percpu_segment has no
stackprotector") disabled the stackprotector for cpu/common.c because of
load_percpu_segment(). Back then the boot stack canary was initialized very
early in start_kernel(). Switching the per CPU area by loading the GDT
caused the stackprotector to fail with paravirt enabled kernels as the
GSBASE was not updated yet. In hindsight a wrong change because it would
have been sufficient to ensure that the canary is the same in both per CPU
areas.

Commit d5553523 ("random: move rand_initialize() earlier") moved the
stack canary initialization to a later point in the init sequence. As a
consequence the per CPU stack canary is 0 when switching the per CPU areas,
so there is no requirement anymore to exclude this file.

Add a comment to load_percpu_segment().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111143.303010511@infradead.org