block: add and use scsi_blk_cmd_ioctl

commit 577ebb37 upstream.

Introduce a wrapper around scsi_cmd_ioctl that takes a block device.

The function will then be enhanced to detect partition block devices
and, in that case, subject the ioctls to whitelisting.

Cc: linux-scsi@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>
Cc: James Bottomley <JBottomley@parallels.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[bwh: Backport to 2.6.32 - adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[wt: slightly changed the interface to match 2.6.27's scsi_cmd_ioctl()
     which still needs the file pointer but has no mode parameter].

Signed-off-by: Willy Tarreau <w@1wt.eu>