hv_balloon: Add support for configurable order free page reporting

hv_balloon: Add support for configurable order free page reporting

Newer versions of Hyper-V allow reporting unused guest pages in chunks
smaller than 2 Mbytes.  Using smaller chunks allows reporting more
unused guest pages, but with increased overhead in the finding the
small chunks.  To make this tradeoff configurable, use the existing
page_reporting_order module parameter to control the reporting order.
Drop and refine checks that restricted the minimun page reporting order
to 2Mbytes size pages. Add appropriate checks to make sure the
underlying Hyper-V versions support cold discard hints of any order
(and not just starting from 9)

Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1664517699-1085-3-git-send-email-shradhagupta@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>