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>
5 files changed