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/loopback/tcm_loop.c b/drivers/target/loopback/tcm_loop.c
index f65dc9d..5491c63 100644
--- a/drivers/target/loopback/tcm_loop.c
+++ b/drivers/target/loopback/tcm_loop.c
@@ -778,7 +778,7 @@
 	 * We now tell TCM to add this WRITE CDB directly into the TCM storage
 	 * object execution queue.
 	 */
-	transport_generic_process_write(se_cmd);
+	target_execute_cmd(se_cmd);
 	return 0;
 }