{pktgen, xfrm} Using "pgset spi xxx" to spedifiy SA for a given flow

User could set specific SPI value to arm pktgen flow with IPsec
transformation, instead of looking up SA by sadr/daddr. The reaseon
to do so is because current state lookup scheme is both slow and, most
important of all, in fact pktgen doesn't need to match any SA state
addresses information, all it needs is the SA transfromation shell to
do the encapuslation.

And this option also provide user an alternative to using pktgen
test existing SA without creating new ones.

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