driver/user: Don't warn for DMA_NONE data direction

Some SCSI commands (for example the TEST UNIT READY command) do not
carry data and so data_direction is DMA_NONE. Patch TCMU to not print a
warning message about unknown data direction, when it is DMA_NONE.

Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Vangelis Koukis <vkoukis@arrikto.com>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>