vdso: Separate vdso_init() on dump/restore

On dump we only need vdso_pfn to be filled,
on restore we need filled symtables.

That means, that we can omit filling symtables on dump,
which also means no need in calling compat_vdso_helper
process with fork(), pipe(), read(), write() and so on syscalls.

Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>