firewire: core: call handler for exclusive regions outside RCU read-side critical section

The previous commit added reference counting to ensure safe invocations of
address handlers.

This commit moves the invocation of handlers for exclusive regions outside
of the RCU read-side critical section. The address handler for the
requested region is selected within the critical section, then invoked
outside of it.

Link: https://lore.kernel.org/r/20250803122015.236493-3-o-takashi@sakamocchi.jp


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