firewire: core: call FCP address handlers outside RCU read-side critical section

The former commit added reference counting to ensure safe invocations of
address handlers. Unlike the exclusive-region address handlers, all FCP
address handlers should be called on receiving an FCP request.

This commit uses the part of kernel stack to collect address handlers up
to 4 within the section, then invoke them outside of the section.
Reference counting ensures that each handler remains valid and safe to
call.

Lifting the limitation of supporting only 4 handlers is left for next
work.

Link: https://lore.kernel.org/r/20250803122015.236493-4-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>