ipvs: flush defense_work before module unload

net/ipv4/ipvs/ip_vs_core.c

	module_exit
	    ip_vs_cleanup
		ip_vs_control_cleanup
		    cancel_rearming_delayed_work
	// done

This is unsafe.  The module may be unloaded and the memory may be freed
while defense_work's handler is still running/preempted.

Do flush_work(&defense_work.work) after cancel_rearming_delayed_work().

Alternatively, we could add flush_work() to cancel_rearming_delayed_work(),
but note that we can't change cancel_delayed_work() in the same manner
because it may be called from atomic context.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>