xfrm: policy: use hlist rcu variants on inexact insert, part 2

This function was modeled on the 'exact' insert one, which did not use
the rcu variant either.

When I fixed the 'exact' insert I forgot to propagate this to my
development tree, so the inexact variant retained the bug.

Fixes: 9cf545eb

 ("xfrm: policy: store inexact policies in a tree ordered by destination address")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>