mnt: Fail collect_mounts when applied to unmounted mounts

The only users of collect_mounts are in audit_tree.c

In audit_trim_trees and audit_add_tree_rule the path passed into
collect_mounts is generated from kern_path passed an audit_tree
pathname which is guaranteed to be an absolute path.   In those cases
collect_mounts is obviously intended to work on mounted paths and
if a race results in paths that are unmounted when collect_mounts
it is reasonable to fail early.

The paths passed into audit_tag_tree don't have the absolute path
check.  But are used to play with fsnotify and otherwise interact with
the audit_trees, so again operating only on mounted paths appears
reasonable.

Avoid having to worry about what happens when we try and audit
unmounted filesystems by restricting collect_mounts to mounts
that appear in the mount tree.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
void __detach_mounts(struct dentry *dentry)
{
	struct mountpoint *mp;
	struct mount *mnt;

	namespace_lock();
	mp = lookup_mountpoint(dentry);
	if (!mp)
		goto out_unlock;

	lock_mount_hash();
	while (!hlist_empty(&mp->m_list)) {
		mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
		umount_tree(mnt, 0);
	}
	unlock_mount_hash();
	put_mountpoint(mp);
out_unlock:
	namespace_unlock();
}

void __detach_mounts(struct dentry *dentry)
{
	struct mountpoint *mp;
	struct mount *mnt;

	namespace_lock();
	mp = lookup_mountpoint(dentry);
	if (!mp)
		goto out_unlock;

	lock_mount_hash();
	while (!hlist_empty(&mp->m_list)) {
		mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
		umount_tree(mnt, 0);
	}
	unlock_mount_hash();
	put_mountpoint(mp);
out_unlock:
	namespace_unlock();
}
