Kernel bugzilla #15239

On some workloads, it is quite possible to get a huge dst list to
process in dst_gc_task(), and trigger soft lockup detection.

Fix is to call cond_resched(), as we run in process context.
Reported-by: NPawel Staszewski <pstaszewski@itcare.pl>
Tested-by: NPawel Staszewski <pstaszewski@itcare.pl>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>