hv_balloon: trace post_status

Hyper-V balloon driver makes non-trivial calculations to convert Linux's
representation of free/used memory to what Hyper-V host expects to see. Add
a tracepoint to see what's being sent and where the data comes from.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>