Preparation for saving vdso_symtable in kdat, which will
allow skip parsing of native and compat symtables - that
at least will save from running compat helper each criu launch.

As {vvar,vdso}_start are randomized with ASLR, there is no
point in saving them into kdat. We'll still need to reread
them from /proc/self/maps for awhile.
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>