Drivers: hv: vmbus: Cleanup hv_post_message()

Minimize failures in this function by pre-allocating the buffer
for posting messages. The hypercall for posting the message can fail
for a number of reasons:

        1. Transient resource related issues
        2. Buffer alignment
        3. Buffer cannot span a page boundry

We address issues 2 and 3 by preallocating a per-cpu page for the buffer.
Transient resource related failures are handled by retrying by the callers
of this function.

This patch is based on the investigation
done by Dexuan Cui <decui@microsoft.com>.

I would like to thank Sitsofe Wheeler <sitsofe@yahoo.com>
for reporting the issue and helping in debuggging.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Cc: <stable@vger.kernel.org>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/hyperv_vmbus.h

@@ -515,6 +515,10 @@ struct hv_context {
 	 * per-cpu list of the channels based on their CPU affinity.
 	 */
 	struct list_head percpu_list[NR_CPUS];
+	/*
+	 * buffer to post messages to the host.
+	 */
+	void *post_msg_page[NR_CPUS];
 };
 
 extern struct hv_context hv_context;