vdso: Separate function for checking vdso's pfn

This is just to split this oversized outgrowed plumped up
parasite_fixup_vdso() and separate it logically.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>