vdso: separate vdso_parse_maps() from vdso_fill_self_symtable()

We need to know compatible vdso/vvar sizes, so add pid argument
to parse another process - it's needed to fork(), remap vdso
and parse child's vdso/vvar in CRIU.

travis-ci: success for Rectify 32-bit compatible C/R on x86
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>