This patch fixes the order in which expectations are evicted when a helper
specifies IP_CT_HELPER_F_REUSE_EXPECT.  Prior to that patch, each new
expectation is added at the beginning of the masters sibling list.  When
max_expected is reached, the first non-established one on the list is reused
(which gave LIFO behaviour).

The patch
- adds new expectations at the end of the masters sibling list
- uses list_for_each_entry for finding the oldes one
- changes nat core to walk backwards throuth the sibling list when calling
  nat helpers
