Revert "ploop_check(): add CHECK_IGNORE_INUSE flag"

The thing is, all read-only images don't have disk-in-use
flag set. So if it is set, we should error out, and this is
what we do.

This reverts commit ece20b22c7efe39ac7acac37f5ae02f8f47006bb.

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