3w-9xxx: don't unmap bounce buffered commands

3w-9xxx: don't unmap bounce buffered commands

3w controller don't dma map small single SGL entry commands but instead
bounce buffer them.  Add a helper to identify these commands and don't
call scsi_dma_unmap for them.

Based on an earlier patch from James Bottomley.

Fixes: 118c85 ("3w-9xxx: fix command completion race")
Reported-by: Tóth Attila <[email protected]>
Tested-by: Tóth Attila <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Acked-by: Adam Radford <[email protected]>
Signed-off-by: James Bottomley <[email protected]>