RDMA/iwcm: Fix memory corruption bug in cm_work_handler()

Possible memory corruption scenario: after putting the work entry back
on the work_free_list, we call process_event() which dereferences
work->event, which could have been modified to another value
meanwhile.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>