mnt: In detach_mounts detach the appropriate unmounted mount

The handling of in detach_mounts of unmounted but connected mounts is
buggy and can lead to an infinite loop.

Correct the handling of unmounted mounts in detach_mount.  When the
mountpoint of an unmounted but connected mount is connected to a
dentry, and that dentry is deleted we need to disconnect that mount
from the parent mount and the deleted dentry.

Nothing changes for the unmounted and connected children.  They can be
safely ignored.

Cc: stable@vger.kernel.org
Fixes: ce07d891 mnt: Honor MNT_LOCKED when detaching mounts
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>