iscsit: use target_execute_cmd for WRITEs

All three callers of transport_generic_handle_data are from user context
and can use target_execute_cmd directly to handle the backend I/O submission
of WRITE I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>