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 <[email protected]>.

I would like to thank Sitsofe Wheeler <[email protected]>
for reporting the issue and helping in debuggging.

Signed-off-by: K. Y. Srinivasan <[email protected]>
Reported-by: Sitsofe Wheeler <[email protected]>
Cc: <[email protected]>
Tested-by: Sitsofe Wheeler <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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