zdtm: check permissions for map_files

Cc: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Test-for: b67d37d96fa0 ("proc_parse: fix vma file open mode recognition")
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>