xfrm: avoid creating temporary SA when there are no listeners

In the case when KMs have no listeners, km_query() will fail and
temporary SAs are garbage collected immediately after their allocation.
This causes strain on memory allocation, leading even to OOM since
temporary SA alloc/free cycle is performed for every packet
and garbage collection does not keep up the pace.

The sane thing to do is to make sure we have audience before
temporary SA allocation.

Signed-off-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>