xen/balloon: Support xend-based toolstack take two

Commit 3aa6c19d ("xen/balloon: Support xend-based toolstack")
tried to fix a regression with running on rather ancient Xen versions.
Unfortunately the fix was based on the assumption that xend would
just use another Xenstore node, but in reality only some downstream
versions of xend are doing that. The upstream xend does not write
that Xenstore node at all, so the problem must be fixed in another
way.

The easiest way to achieve that is to fall back to the behavior
before commit 96edd61d ("xen/balloon: don't online new memory
initially") in case the static memory maximum can't be read.

This is achieved by setting static_max to the current number of
memory pages known by the system resulting in target_diff becoming
zero.

Fixes: 3aa6c19d ("xen/balloon: Support xend-based toolstack")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: <stable@vger.kernel.org> # 4.13
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-balloon.c

@@ -94,7 +94,7 @@ static void watch_target(struct xenbus_watch *watch,
 				  "%llu", &static_max) == 1))
 			static_max >>= PAGE_SHIFT - 10;
 		else
-			static_max = new_target;
+			static_max = balloon_stats.current_pages;
 
 		target_diff = (xen_pv_domain() || xen_initial_domain()) ? 0
 				: static_max - balloon_stats.target_pages;