free_images_list(): remove

Function free_images_list() does the same as ploop_free_array().
As the latter one is in our API, let's use it everythere and
ditch the former.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
check_deltas(): read-only check for non-top deltas

This partially reverts commit f5e4719. Opening non-top deltas
in rw mode can actually be dangerous and is not really expected
by user, as those deltas are supposed to be used in read-only mode
and therefore should stay intact.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>