block: Fail gracefully when blockdev-snapshot creates loops

block: Fail gracefully when blockdev-snapshot creates loops

Using blockdev-snapshot to append a node as an overlay to itself, or to
any of its parents, causes crashes. Catch the condition and return an
error for these cases instead.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1824363
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211018134714.48438-1-kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>