xfrm: policy: return NULL when inexact search needed

currently policy_hash_bysel() returns the hash bucket list
(for exact policies), or the inexact list (when policy uses a prefix).

Searching this inexact list is slow, so it might be better to pre-sort
inexact lists into a tree or another data structure for faster
searching.

However, due to 'any' policies, that need to be searched in any case,
doing so will require that 'inexact' policies need to be handled
specially to decide the best search strategy.  So change hash_bysel()
and return NULL if the policy can't be handled via the policy hash
table.

Right now, we simply use the inexact list when this happens, but
future patch can then implement a different strategy.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>