nvmet: auth: use NULL to clear a pointer in nvmet_auth_sq_free()

When compiling with C=1, the following sparse warning is generated:

auth.c:243:23: warning: Using plain integer as NULL pointer

Avoid this warning by using NULL to instead of 0 to set the sq tls_key
pointer.

Fixes: fa2e0f8bbc ("nvmet-tcp: support secure channel concatenation")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/auth.c

@@ -240,7 +240,7 @@ void nvmet_auth_sq_free(struct nvmet_sq *sq)
 {
 	cancel_delayed_work(&sq->auth_expired_work);
 #ifdef CONFIG_NVME_TARGET_TCP_TLS
-	sq->tls_key = 0;
+	sq->tls_key = NULL;
 #endif
 	kfree(sq->dhchap_c1);
 	sq->dhchap_c1 = NULL;