Pull mount propagation fix from Al Viro:
"6.15 allowed mount propagation to destinations in detached trees;
unfortunately, that breaks existing userland, so the old behaviour
needs to be restored.
It's not exactly a revert - the original behaviour had a bug, where
existence of detached tree might disrupt propagation between locations
not in detached trees. Thankfully, userland did not depend upon that
bug, so we want to keep the fix"
* tag 'pull-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
Don't propagate mounts into detached trees