vdso: Move parsing of self/maps outside vdso_fill_self_symtable()

As ASLR randomizes {vdso,vvar}_start between criu launches,
vdso_parse_maps() should be called each launch:
- on restore to know {vdso,vvar}_start position for later parking
  in restorer's save zone
- on checkpointing to get vdso's pfn for pre-v3.16 kernels
  which lose "[vdso]" hint in maps file.

But vdso_fill_symtable() call may be omitted if symtable is
inside kdat file.

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