util: epoll: move comment about timeout decrease to uffd.c

The generic epoll_wait wrapper should not do any assumptions about timeout.
It's it up to lazy-pages daemon to make (future) policy decisions.

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