xfrm: Fix refcount imbalance in xfrm_lookup

xfrm_lookup must return a dst_entry with a refcount for the caller.
Git commit 1a1ccc96 ("xfrm: Remove caching of xfrm_policy_sk_bundles")
removed this refcount for the socket policy case accidentally.
This patch restores it and sets DST_NOCACHE flag to make sure
that the dst_entry is freed when the refcount becomes null.

Fixes: 1a1ccc96 ("xfrm: Remove caching of xfrm_policy_sk_bundles")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>