Drivers: hv: vmbus: Add a helper function is_sub_channel()

The existing method of telling if a channel is sub-channel in
vmbus_process_offer() is cumbersome. This new simple helper function
is preferred in future.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/linux/hyperv.h

@@ -245,7 +245,10 @@ struct vmbus_channel_offer {
 		} pipe;
 	} u;
 	/*
-	 * The sub_channel_index is defined in win8.
+	 * The sub_channel_index is defined in Win8: a value of zero means a
+	 * primary channel and a value of non-zero means a sub-channel.
+	 *
+	 * Before Win8, the field is reserved, meaning it's always zero.
 	 */
 	u16 sub_channel_index;
 	u16 reserved3;
@@ -934,6 +937,11 @@ static inline bool is_hvsock_channel(const struct vmbus_channel *c)
 		  VMBUS_CHANNEL_TLNPI_PROVIDER_OFFER);
 }
 
+static inline bool is_sub_channel(const struct vmbus_channel *c)
+{
+	return c->offermsg.offer.sub_channel_index != 0;
+}
+
 static inline void set_channel_affinity_state(struct vmbus_channel *c,
 					      enum hv_numa_policy policy)
 {