xfrm: Fix state threshold configuration from userspace

Allow to change the replay threshold (XFRMA_REPLAY_THRESH) and expiry
timer (XFRMA_ETIMER_THRESH) of a state without having to set other
attributes like replay counter and byte lifetime. Changing these other
values while traffic flows will break the state.

Signed-off-by: Michael Rossberg <michael.rossberg@tu-ilmenau.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>