fix race that can give duplicate host number

Just once, two fcoe instances got the same host number
from scsi_add_host().

Use atomic_t and atomic_inc_return() to get next host number.
Subtract 1, so that scsi_host still starts with 0.

[jejb: added comment about unusual subtraction]
Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/hosts.c

@@ -40,7 +40,7 @@
 #include "scsi_logging.h"
 
 
-static int scsi_host_next_hn;		/* host_no for next new host */
+static atomic_t scsi_host_next_hn;	/* host_no for next new host */
 
 
 static void scsi_host_cls_release(struct device *dev)
@@ -333,7 +333,11 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 
 	mutex_init(&shost->scan_mutex);
 
-	shost->host_no = scsi_host_next_hn++; /* XXX(hch): still racy */
+	/*
+	 * subtract one because we increment first then return, but we need to
+	 * know what the next host number was before increment
+	 */
+	shost->host_no = atomic_inc_return(&scsi_host_next_hn) - 1;
 	shost->dma_channel = 0xff;
 
 	/* These three are default values which can be overridden */