scsi: core: Handle drivers which set sg_tablesize to zero

In scsi_mq_setup_tags(), cmd_size is calculated based on zero size for the
scatter-gather list in case the low level driver uses SG_NONE in its host
template.

cmd_size is passed on to the block layer for calculation of the request
size, and we've seen NULL pointer dereference errors from the block layer
in drivers where SG_NONE is used and a mq IO scheduler is active,
apparently as a consequence of this (see commit 68ab2d76

 ("scsi:
cxlflash: Set sg_tablesize to 1 instead of SG_NONE"), and a recent patch by
Finn Thain converting the three m68k NFR5380 drivers to avoid setting
SG_NONE).

Try to avoid these errors by accounting for at least one sg list entry when
calculating cmd_size, regardless of whether the low level driver set a zero
sg_tablesize.

Tested on 030 m68k with the atari_scsi driver - setting sg_tablesize to
SG_NONE no longer results in a crash when loading this driver.

CC: Finn Thain <fthain@telegraphics.com.au>
Link: https://lore.kernel.org/r/1572922150-4358-1-git-send-email-schmitzmic@gmail.com
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>