do_move_mount(): take dropping the old mountpoint into attach_recursive_mnt()

do_move_mount(): take dropping the old mountpoint into attach_recursive_mnt()

... and fold it with unhash_mnt() there - there's no need to retain a reference
to old_mp beyond that point, since by then all mountpoints we were going to add
are either explicitly pinned by get_mountpoint() or have stuff already added
to them.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>