nvmet-fc: use RCU list iterator for assoc_list

The assoc_list is a RCU protected list, thus use the RCU flavor of list
functions.

Let's use this opportunity and refactor this code and move the lookup
into a helper and give it a descriptive name.

Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>