RDMA/cxgb4: Fix endpoint timeout race condition

The endpoint timeout logic had a race that could cause an endpoint
object to be freed while it was still on the timedout list.  This
can happen if the timer is stopped after it had fired, but before
the timedout thread processed the endpoint timeout.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>