iscsi-target: Fix snprintf usage with MAX_PORTAL_LEN

This patch makes lio_target_call_addnptotpg() use sprintf() with
MAX_PORTAL_LEN + 1 to address the following smatch warning:

drivers/target/iscsi/iscsi_target_configfs.c +184 lio_target_call_addnptotpg(21)
        error: snprintf() chops off the last chars of 'name': 257 vs 256
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_configfs.c

@@ -181,7 +181,7 @@ struct se_tpg_np *lio_target_call_addnptotpg(
 		return ERR_PTR(-EOVERFLOW);
 	}
 	memset(buf, 0, MAX_PORTAL_LEN + 1);
-	snprintf(buf, MAX_PORTAL_LEN, "%s", name);
+	snprintf(buf, MAX_PORTAL_LEN + 1, "%s", name);
 
 	memset(&sockaddr, 0, sizeof(struct __kernel_sockaddr_storage));