dst: call cond_resched() in dst_gc_task()

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>

net/core/dst.c

@@ -17,6 +17,7 @@
 #include <linux/string.h>
 #include <linux/types.h>
 #include <net/net_namespace.h>
+#include <linux/sched.h>
 
 #include <net/dst.h>
 
@@ -79,6 +80,7 @@ static void dst_gc_task(struct work_struct *work)
 	while ((dst = next) != NULL) {
 		next = dst->next;
 		prefetch(&next->next);
+		cond_resched();
 		if (likely(atomic_read(&dst->__refcnt))) {
 			last->next = dst;
 			last = dst;