uffd: do not treat VDSO pages as lazy

VDSO is just a few pages and they can be loaded directly rather than go
through userfaultfd to save some complexity on the lazy-pages daemon side.

Signed-off-by: Mike Rapoport <rapoport@il.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>