criu: dedup/pagemap: move most of the dedup code to pagemap.c

The deduplication code uses pagemap internal implementation details, so
it's better to have this code in the pagemap.c

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>