[SCSI] add support for per-host cmd pools

This allows drivers to specify the size of their per-command private
data in the host template and then get extra memory allocated for
each command instead of needing another allocation in ->queuecommand.

With the current SCSI code that already does multiple allocations for
each command this probably doesn't make a big performance impact, but
it allows to clean up the drivers, and prepare them for using the
blk-mq infrastructure where the common allocation will make a difference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

include/scsi/scsi_host.h

@@ -15,6 +15,7 @@ struct completion;
 struct module;
 struct scsi_cmnd;
 struct scsi_device;
+struct scsi_host_cmd_pool;
 struct scsi_target;
 struct Scsi_Host;
 struct scsi_host_cmd_pool;
@@ -524,6 +525,12 @@ struct scsi_host_template {
 	 *   scsi_netlink.h
 	 */
 	u64 vendor_id;
+
+	/*
+	 * Additional per-command data allocated for the driver.
+	 */
+	unsigned int cmd_size;
+	struct scsi_host_cmd_pool *cmd_pool;
 };
 
 /*