mnt: In umount_tree reuse mnt_list instead of mnt_hash

mnt: In umount_tree reuse mnt_list instead of mnt_hash

commit c003b26ff98ca04a180ff34c38c007a3998d62f9 upstream.

umount_tree builds a list of mounts that need to be unmounted.
Utilize mnt_list for this purpose instead of mnt_hash.  This begins to
allow keeping a mount on the mnt_hash after it is unmounted, which is
necessary for a properly functioning MNT_LOCKED implementation.

The fact that mnt_list is an ordinary list makding available list_move
is nice bonus.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>