ib mad: fix an incorrect use of list_for_each_entry

In tf_dequeue(), if we reach the end of the list without finding a
non-cancelled element, "tmp" will be a pointer into the list head, so the
tmp->canceled check is bogus. Use a flag instead.

Submitted by: Tao Liu <Tao.Liu@isilon.com>
Reviewed by: hselasky
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3244