scsi: sd: Fix cache_type_store()

Changing of caching mode via /sys/devices/.../scsi_disk/.../cache_type may
fail if device responds to MODE SENSE command with DPOFUA flag set, and
then checks this flag to be not set on MODE SELECT command.

In this scenario, when trying to change cache_type, write always fails:

	# echo "none" >cache_type
	bash: echo: write error: Invalid argument

And following appears in dmesg:

	[13007.865745] sd 1:0:1:0: [sda] Sense Key : Illegal Request [current]
	[13007.865753] sd 1:0:1:0: [sda] Add. Sense: Invalid field in parameter list

From SBC-4 r15, 6.5.1 "Mode pages overview", description of DEVICE-SPECIFIC
PARAMETER field in the mode parameter header:
	...
	The write protect (WP) bit for mode data sent with a MODE SELECT
	command shall be ignored by the device server.
	...
	The DPOFUA bit is reserved for mode data sent with a MODE SELECT
	command.
	...

The remaining bits in the DEVICE-SPECIFIC PARAMETER byte are also reserved
and shall be set to zero.

[mkp: shuffled commentary to commit description]

Cc: stable@vger.kernel.org
Signed-off-by: Ivan Mironov <mironov.ivan@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>