cleanup SCSI ioctl support v2

Hi all,

this series cleans up the scsi ioctl handler and merges the "block layer"
SCSI ioctl code that is only used by the SCSI layer and its drivers now
into the main SCSI ioctl handler.

Changes since v1:
 - replace and ifdef with an IS_ENABLED check to cater for modular bsg
   builds
 - rename queue_max_sectors_bytes to queue_max_bytes
 - return an unsigned value from queue_max_bytes
 - rename a few symbols to match their move to the SCSI code
 - scsi_common.c needs module.h for some configs, so add іt
 - use an u8 array instead of a char one for the CDB
 - document scsi_cmd_allowed

Diffstat: