tick/nohz: unexport __init-annotated tick_nohz_full_setup()

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

tick/nohz: unexport __init-annotated tick_nohz_full_setup()

EXPORT_SYMBOL and __init is a bad combination because the .init.text

section is freed up after the initialization. Hence, modules cannot

use symbols annotated __init. The access to a freed symbol may end up

with kernel panic.

modpost used to detect it, but it had been broken for a decade.

Commit 28438794aba4 ("modpost: fix section mismatch check for exported

init/exit sections") fixed it so modpost started to warn it again, then

this showed up:

MODPOST vmlinux.symvers

WARNING: modpost: vmlinux.o(___ksymtab_gpl+tick_nohz_full_setup+0x0): Section mismatch in reference from the variable __ksymtab_tick_nohz_full_setup to the function .init.text:tick_nohz_full_setup()

The symbol tick_nohz_full_setup is exported and annotated __init

Fix this by removing the __init annotation of tick_nohz_full_setup or drop the export.

Drop the export because tick_nohz_full_setup() is only called from the

built-in code in kernel/sched/isolation.c.