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: ca925cf1 ("flowcache: Make flow cache name space aware")
Reported-by: Tomáš Charvát <tc@excello.cz>
Tested-by: Jan Hejl <jh@excello.cz>
Signed-off-by: Miroslav Urbanek <mu@miroslavurbanek.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>