vdso: Don't read pagemap or parse symtable under vdso_hint_reliable

We can save some syscalls for *each* dumpee if we don't
open()/seek()/read()/close() /proc/pid/pagemap for each
dumpee and even don't use parasite to parse symtable if
pagemap is unavailable.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>