commit bdddbf69 upstream.

The returned xfrm_state should be hold before unlock xfrm_state_lock,
otherwise the returned xfrm_state maybe be released.

Fixes: c454997e[{pktgen, xfrm} Introduce xfrm_state_lookup_byspi..]
Cc: Fan Du <fan.du@intel.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Fan Du <fan.du@intel.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>