print_page_stats: Reserve 7 digits for page count alignment

print_page_stats: Reserve 7 digits for page count alignment

If there are mappings that are larger than 4 gigabytes, 6 digits is
not enough for the output. 7 digits is enough until 40 gigabytes,
which should probably be enough in most of users. If someone has got
more than 40 gigabytes of ram and wants to have nicely aligned page
usage prints, I'm happy to implement some sort of dynamic
alignmenting for it..

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>