[SCSI] add shared tag map helpers

This patch adds support for sharing tag maps at the host level
(i.e. either every queue [LUN] has its own tag map or there's a single
one for the entire host).  This formulation is primarily intended to
help single issue queue hardware, like the aic7xxx

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

include/scsi/scsi_host.h

@@ -16,6 +16,7 @@ struct scsi_target;
 struct Scsi_Host;
 struct scsi_host_cmd_pool;
 struct scsi_transport_template;
+struct blk_queue_tags;
 
 
 /*
@@ -465,6 +466,12 @@ struct Scsi_Host {
 	struct scsi_host_template *hostt;
 	struct scsi_transport_template *transportt;
 
+	/*
+	 * area to keep a shared tag map (if needed, will be
+	 * NULL if not)
+	 */
+	struct blk_queue_tag	*bqt;
+
 	/*
 	 * The following two fields are protected with host_lock;
 	 * however, eh routines can safely access during eh processing