{pktgen, xfrm} Introduce xfrm_state_lookup_byspi for pktgen

Introduce xfrm_state_lookup_byspi to find user specified by custom
from "pgset spi xxx". Using this scheme, any flow regardless its
saddr/daddr could be transform by SA specified with configurable
spi.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>