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: Pawel Staszewski <[e-mail]>
Tested-by: Pawel Staszewski <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/dst.c

···
 #include <linux/string.h>
 #include <linux/types.h>
 #include <net/net_namespace.h>
+#include <linux/sched.h>
 
 #include <net/dst.h>
 
···
 	while ((dst = next) != NULL) {
 		next = dst->next;
 		prefetch(&next->next);
+		cond_resched();
 		if (likely(atomic_read(&dst->__refcnt))) {
 			last->next = dst;
 			last = dst;