module: Begin to move from RCU-sched to RCU.

The RCU usage in module was introduced in commit d72b37513cdfb ("Remove
stop_machine during module load v2") and it claimed not to be RCU but
similar. Then there was another improvement in commit e91defa26c527
("module: don't use stop_machine on module load"). It become a mix of
RCU and RCU-sched and was eventually fixed 0be964be0d450 ("module:
Sanitize RCU usage and locking"). Later RCU & RCU-sched was merged in
commit cb2f55369d3a9 ("modules: Replace synchronize_sched() and
call_rcu_sched()") so that was aligned.

Looking at it today, there is still leftovers. The preempt_disable() was
used instead rcu_read_lock_sched(). The RCU & RCU-sched merge was not
complete as there is still rcu_dereference_sched() for module::kallsyms.

The RCU-list modules and unloaded_tainted_modules are always accessed
under RCU protection or the module_mutex. The modules list iteration can
always happen safely because the module will not disappear.
Once the module is removed (free_module()) then after removing the
module from the list, there is a synchronize_rcu() which waits until
every RCU reader left the section. That means iterating over the list
within a RCU-read section is enough, there is no need to disable
preemption. module::kallsyms is first assigned in add_kallsyms() before
the module is added to the list. At this point, it points to init data.
This pointer is later updated and before the init code is removed there
is also synchronize_rcu() in do_free_init(). That means A RCU read lock
is enough for protection and rcu_dereference() can be safely used.

Convert module code and its users step by step. Update comments and
convert print_modules() to use RCU.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250108090457.512198-3-bigeasy@linutronix.de
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>

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