lazy-pages: add comments about different addresses in lazy_iov and lp_req

Both lazy_iov and lp_req have two fields for address/start: the run-time
address that tracks remaps, and the "dump time" address, which is required
for pagemap accesses.

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