While doing pre-dump we don't do proper VDSO fixups, thus at
this stage we may fail the should_dump_page() checks -- it
will tread VDSO pages are 'regular' and may skip dumping some
of them.

This is not bad as is, but the subsequent dump will properly
spot VDSO are and will try to dump _all_ pages from it. And
if checks for soft-dirty will report that some pages are clean,
dump will try to locate those in parent images and would fail.
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>