[SCSI] sg: cap reserved_size values at max_sectors

This patch (as857) modifies the SG_GET_RESERVED_SIZE and
SG_SET_RESERVED_SIZE ioctls in the sg driver, capping the values at
the device's request_queue's max_sectors value.  This will permit
cdrecord to obtain a legal value for the maximum transfer length,
fixing Bugzilla #7026.

The patch also caps the initial reserved_size value.  There's no
reason to have a reserved buffer larger than max_sectors, since it
would be impossible to use the extra space.

The corresponding ioctls in the block layer are modified similarly,
and the initial value for the reserved_size is set as large as
possible.  This will effectively make it default to max_sectors.
Note that the actual value is meaningless anyway, since block devices
don't have a reserved buffer.

Finally, the BLKSECTGET ioctl is added to sg, so that there will be a
uniform way for users to determine the actual max_sectors value for
any raw SCSI transport.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

block/ll_rw_blk.c

@@ -1925,6 +1925,8 @@ blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
 	blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
 	blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
 
+	q->sg_reserved_size = INT_MAX;
+
 	/*
 	 * all done
 	 */

block/scsi_ioctl.c

@@ -78,7 +78,9 @@ static int sg_set_timeout(request_queue_t *q, int __user *p)
 
 static int sg_get_reserved_size(request_queue_t *q, int __user *p)
 {
-	return put_user(q->sg_reserved_size, p);
+	unsigned val = min(q->sg_reserved_size, q->max_sectors << 9);
+
+	return put_user(val, p);
 }
 
 static int sg_set_reserved_size(request_queue_t *q, int __user *p)