xen/balloon: Use PageHighMem() for high memory page detection

Replace pfn < max_low_pfn by !PageHighMem() in increase_reservation().
It makes more clearer what is going on.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>