malloc: Fix list_lock/arena lock deadlock [BZ #19182]

	* malloc/arena.c (list_lock): Document lock ordering requirements.
	(free_list_lock): New lock.
	(ptmalloc_lock_all): Comment on free_list_lock.
	(ptmalloc_unlock_all2): Reinitialize free_list_lock.
	(detach_arena): Update comment.  free_list_lock is now needed.
	(_int_new_arena): Use free_list_lock around detach_arena call.
	Acquire arena lock after list_lock.  Add comment, including FIXME
	about incorrect synchronization.
	(get_free_list): Switch to free_list_lock.
	(reused_arena): Acquire free_list_lock around detach_arena call
	and attached threads counter update.  Add two FIXMEs about
	incorrect synchronization.
	(arena_thread_freeres): Switch to free_list_lock.
	* malloc/malloc.c (struct malloc_state): Update comments to
	mention free_list_lock.