xfrm: policy: fix infinite loop when merging src-nodes

With very small change to test script we can trigger softlockup due to
bogus assignment of 'p' (policy to be examined) on restart.

Previously the two to-be-merged nodes had same address/prefixlength pair,
so no erase/reinsert was necessary, we only had to append the list from
node a to b.

If prefix lengths are different, the node has to be deleted and re-inserted
into the tree, with the updated prefix length.  This was broken; due to
bogus update to 'p' this loops forever.

Add a 'restart' label and use that instead.

While at it, don't perform the unneeded reinserts of the policies that
are already sorted into the 'new' node.

A previous patch in this series made xfrm_policy_inexact_list_reinsert()
use the relative position indicator to sort policies according to age in
case priorities are identical.

Fixes: 6ac098b2

 ("xfrm: policy: add 2nd-level saddr trees for inexact policies")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>