vdso/kdat: Store symtable in kerndat_s

Don't need to parse vdso symtable each restore - it's boot-persistent,
so move it into criu.kdat file on tmpfs.
That will also remove syscalls made for filling compat vdso symtable
by compat vdso helper.

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