IB/mad: Fix incorrect access to items on local_list

In cancel_mads(), MADs are moved from the wait_list and local_list
to a cancel_list for processing.  However, the structures on these two
lists are not the same.  The wait_list references struct
ib_mad_send_wr_private, but local_list references struct
ib_mad_local_private.  Cancel_mads() treats all items moved to the
cancel_list as struct ib_mad_send_wr_private.  This leads to a system
crash when requests are moved from the local_list to the cancel_list.

Fix this by leaving local_list alone.  All requests on the local_list
have completed are just awaiting processing by a queued worker thread.

Bug (crash) reported by Dotan Barak <dotanb@dev.mellanox.co.il>.
Problem with local_list access reported by Robert Reynolds
<rreynolds@opengridcomputing.com>.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>