scsi: target: Don't request modules that aren't even built

If, for example, I don't enable CONFIG_TCM_PSCSI, then every time I load
the target subsystem, I get an annoying

    Unable to load target_core_pscsi

kernel log message.  Instead let's only request_module() on things if
that code is enabled.
Signed-off-by: NRoland Dreier <roland@purestorage.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_transport.c

@@ -205,19 +205,19 @@ void transport_subsystem_check_init(void)
 	if (sub_api_initialized)
 		return;
 
-	ret = request_module("target_core_iblock");
+	ret = IS_ENABLED(CONFIG_TCM_IBLOCK) && request_module("target_core_iblock");
 	if (ret != 0)
 		pr_err("Unable to load target_core_iblock\n");
 
-	ret = request_module("target_core_file");
+	ret = IS_ENABLED(CONFIG_TCM_FILEIO) && request_module("target_core_file");
 	if (ret != 0)
 		pr_err("Unable to load target_core_file\n");
 
-	ret = request_module("target_core_pscsi");
+	ret = IS_ENABLED(CONFIG_TCM_PSCSI) && request_module("target_core_pscsi");
 	if (ret != 0)
 		pr_err("Unable to load target_core_pscsi\n");
 
-	ret = request_module("target_core_user");
+	ret = IS_ENABLED(CONFIG_TCM_USER2) && request_module("target_core_user");
 	if (ret != 0)
 		pr_err("Unable to load target_core_user\n");