[SCSI] mptsas: support basic hotplug

Adds hotplug support for SAS end devices.  Unfortunately the fusion
firmware doesn't generate similar events for expanders addition/removal
so we can't support them yet.  Eric has an idea about a clever scheme to
find out about expander changes so that'll be added later on.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/message/fusion/mptbase.h

@@ -612,6 +612,7 @@ typedef struct _MPT_ADAPTER
 	struct list_head	 list;
 	struct net_device	*netdev;
 	struct list_head	 sas_topology;
+	struct mutex		 sas_topology_mutex;
 	MPT_SAS_MGMT		 sas_mgmt;
 } MPT_ADAPTER;