nvmet-fc: check successful reference in nvmet_fc_find_target_assoc

When searching for an association based on an association id, when there
is a match, the code takes a reference. However, it is not validating
that the reference taking was successful.

Check the status of the reference. If unsuccessful, the device is being
deleted and should be ignored.
Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/fc.c

@@ -1243,7 +1243,8 @@ nvmet_fc_find_target_assoc(struct nvmet_fc_tgtport *tgtport,
 	list_for_each_entry(assoc, &tgtport->assoc_list, a_list) {
 		if (association_id == assoc->association_id) {
 			ret = assoc;
-			nvmet_fc_tgt_a_get(assoc);
+			if (!nvmet_fc_tgt_a_get(assoc))
+				ret = NULL;
 			break;
 		}
 	}