Previously, arch_prctl(ARCH_MAP_VDSO_32) was only used by
CONFIG_COMPAT to map compatible vdso blob for 32-bit restoree.
But we can make it more generic:
Omitting mremap() for moving vdso to rt-vdso zone in restorer
and afterward on needed position in restoree.
Also omitting reading /proc/self/maps to find vdso/vvar
addresses (to park afterward in restorer).

TLDR; under this kdat feature we can get rid of a buch of mremap()'s
for each restoree and from parsing /proc/self/maps in vdso_init_restore().

The API is present from v4.9 kernel.
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>