nvme-fabrics: get a reference when reusing a nvme_host structure

Without this we'll get a use after free after connecting two controller
using the same hostnqn and then disconnecting one of them.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>

drivers/nvme/host/fabrics.c

@@ -47,8 +47,10 @@ static struct nvmf_host *nvmf_host_add(const char *hostnqn)
 
 	mutex_lock(&nvmf_hosts_mutex);
 	host = __nvmf_host_find(hostnqn);
-	if (host)
+	if (host) {
+		kref_get(&host->ref);
 		goto out_unlock;
+	}
 
 	host = kmalloc(sizeof(*host), GFP_KERNEL);
 	if (!host)