[IPSEC]: Fix strange IPsec freeze.

Problem discovered and initial patch by Olaf Kirch:

	there's a problem with IPsec that has been bugging some of our users
	for the last couple of kernel revs. Every now and then, IPsec will
	freeze the machine completely. This is with openswan user land,
	and with kernels up to and including 2.6.16-rc2.

	I managed to debug this a little, and what happens is that we end
	up looping in xfrm_lookup, and never get out. With a bit of debug
	printks added, I can this happening:

		ip_route_output_flow calls xfrm_lookup

		xfrm_find_bundle returns NULL (apparently we're in the
			middle of negotiating a new SA or something)

		We therefore call xfrm_tmpl_resolve. This returns EAGAIN
			We go to sleep, waiting for a policy update.
			Then we loop back to the top

		Apparently, the dst_orig that was passed into xfrm_lookup
			has been dropped from the routing table (obsolete=2)
			This leads to the endless loop, because we now create
			a new bundle, check the new bundle and find it's stale
			(stale_bundle -> xfrm_bundle_ok -> dst_check() return 0)

	People have been testing with the patch below, which seems to fix the
	problem partially. They still see connection hangs however (things
	only clear up when they start a new ping or new ssh). So the patch
	is obvsiouly not sufficient, and something else seems to go wrong.

	I'm grateful for any hints you may have...

I suggest that we simply bail out always.  If the dst decides to die
on us later on, the packet will be dropped anyway.  So there is no
great urgency to retry here.  Once we have the proper resolution
queueing, we can then do the retry again.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Olaf Kirch <okir@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>