nvmet-fc: add a missing __rcu annotation to nvmet_fc_tgt_assoc.queues

Make sparse happy after the recent conversion to RCU lookups.

Fixes: 4e2f02bf

 ("nvmet-fc: use RCU proctection for assoc_list")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: James Smart <james.smart@broadcom.com>