hpsa: count passthru cmds with atomics, not a spin locked int

Performance enhancement. Remove spin_locks from the driver.

Reviewed-by: Scott Teel <scott.teel@pmcs.com>
Signed-off-by: Don Brace <don.brace@pmcs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/drivers/scsi/hpsa.h b/drivers/scsi/hpsa.h
index 981479a1..1856445 100644
--- a/drivers/scsi/hpsa.h
+++ b/drivers/scsi/hpsa.h
@@ -183,8 +183,7 @@
 
 	/* cap concurrent passthrus at some reasonable maximum */
 #define HPSA_MAX_CONCURRENT_PASSTHRUS (10)
-	spinlock_t passthru_count_lock; /* protects passthru_count */
-	int passthru_count;
+	atomic_t passthru_cmds_avail;
 
 	/*
 	 * Performant mode completion buffers