flowcache: Fix kernel panic in flow_cache_flush_task

flow_cache_flush_task references a structure member flow_cache_gc_work
where it should reference flow_cache_flush_task instead.

Kernel panic occurs on kernels using IPsec during XFRM garbage
collection. The garbage collection interval can be shortened using the
following sysctl settings:

net.ipv4.xfrm4_gc_thresh=4
net.ipv6.xfrm6_gc_thresh=4

With the default settings, our productions servers crash approximately
once a week. With the settings above, they crash immediately.

Fixes: ca925cf1534e ("flowcache: Make flow cache name space aware")
Reported-by: Tomáš Charvát <[e-mail]>
Tested-by: Jan Hejl <[e-mail]>
Signed-off-by: Miroslav Urbanek <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/flow.c

···
 static void flow_cache_flush_task(struct work_struct *work)
 {
 	struct netns_xfrm *xfrm = container_of(work, struct netns_xfrm,
-						flow_cache_gc_work);
+						flow_cache_flush_work);
 	struct net *net = container_of(xfrm, struct net, xfrm);
 
 	flow_cache_flush(net);