[XFRM]: Fix oops in xfrm4_dst_destroy()

With 2.6.21-rc1, I get an oops when running 'ifdown eth0' and an IPsec
connection is active. If I shut down the connection before running 'ifdown
eth0', then there's no problem.  The critical operation of this script is to
kill dhcpd.

The problem is probably caused by commit with git identifier
43372262 (Linus tree) "[IPSEC]: IPv4 over IPv6
IPsec tunnel".

This patch fixes that oops. I don't know the network code of the Linux
kernel in deep, so if that fix is wrong, please change it. But please
fix the oops. :)

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>