[XFRM]: Fix crash introduced by struct dst_entry reordering

XFRM expects xfrm_dst->u.next to be same pointer as dst->next, which
was broken by the dst_entry reordering in commit 1e19e02c~, causing
an oops in xfrm_bundle_ok when walking the bundle upwards.

Kill xfrm_dst->u.next and change the only user to use dst->next instead.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>