sbp-target: fix return value check in sbp_register_configfs()

In case of error, the function target_fabric_configfs_init() returns
ERR_PTR() not NULL pointer. The NULL test in the return value check
should be replaced with IS_ERR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/sbp/sbp_target.c

@@ -2543,9 +2543,9 @@ static int sbp_register_configfs(void)
 	int ret;
 
 	fabric = target_fabric_configfs_init(THIS_MODULE, "sbp");
-	if (!fabric) {
+	if (IS_ERR(fabric)) {
 		pr_err("target_fabric_configfs_init() failed\n");
-		return -ENOMEM;
+		return PTR_ERR(fabric);
 	}
 
 	fabric->tf_ops = sbp_ops;