vdso/compat: Don't unmap missing vdso/vvar vmas

I've met missing vvar on Virtuozzo 7 kernel - just skip
unmapping it.

TODO: check ia32 C/R with kernel CONFIG_VDSO=n

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