datapath: Avoid deadlock on dp_mutex versus kthread_stop().

datapath: Avoid deadlock on dp_mutex versus kthread_stop().

When a datapath is deleted, del_dp() acquires dp_mutex and uses
kthread_stop() to wait for the dp_task to die.  Meanwhile, dp_task may
be waiting to acquire dp_mutex and won't die until it does so.

This commit fixes the problem by sending SIGKILL to dp_task before
call kthread_stop() and making dp_task give up if it receives a signal.