mmu: fix offset calculation in page mapper

Generic page mapper code provides offset to mapper's small/huge_page
callbacks, but to make this offset meaningful it should be an offset
from a beginning of a vma that is mapped, but currently it is an offset
from a starting address of current mapping operation which means it
will be always zero it mapping is done page by page. Fix this by passing
starting vma address to page mapper for correct offset calculation.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>