scsi: Improve CDL control

With ATA devices supporting the CDL feature, using CDL requires that the
feature be enabled with a SET FEATURES command. This command is issued
as the translated command for the MODE SELECT command issued by
scsi_cdl_enable() when the user enables CDL through the device
cdl_enable sysfs attribute.

However, the implementation of scsi_cdl_enable() always issues a MODE
SELECT command for ATA devices when the enable argument is true, even if
CDL is already enabled on the device. While this does not cause any
issue with using CDL descriptors with read/write commands (the CDL
feature will be enabled on the drive), issuing the MODE SELECT command
even when the device CDL feature is already enabled will cause a reset
of the ATA device CDL statistics log page (as defined in ACS, any CDL
enable action must reset the device statistics).

Avoid this needless actions (and the implied statistics log page reset)
by modifying scsi_cdl_enable() to issue the MODE SELECT command to
enable CDL if and only if CDL is not reported as already enabled on the
device.

And while at it, simplify the initialization of the is_ata boolean
variable and move the declaration of the scsi mode data and sense header
variables to within the scope of ATA device handling.

Fixes: 1b22cfb1 ("scsi: core: Allow enabling and disabling command duration limits")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Igor Pylypiv <ipylypiv@google.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>