target: remove transport_generic_process_write

Just call target_execute_cmd directly.  Also, convert loopback, sbp,
usb-gadget to use the newly exported target_execute_cmd().

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/target/sbp/sbp_target.c b/drivers/target/sbp/sbp_target.c
index 7e6136e..0848c4c 100644
--- a/drivers/target/sbp/sbp_target.c
+++ b/drivers/target/sbp/sbp_target.c
@@ -1784,8 +1784,7 @@
 		return ret;
 	}
 
-	transport_generic_process_write(se_cmd);
-
+	target_execute_cmd(se_cmd);
 	return 0;
 }