mem: Don't do unneeded mprotects

When a vma we restore doesn't have any pages in pagemaps there's
not need to enforce PROT_WRITE bit on it.

This only applies to non-premmaped vmas.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>