xfrm: simplify SA initialization routine

SA replay mode is initialized differently for user-space and
kernel-space users, but the call to xfrm_init_replay() existed in
common path with boolean protection. That caused to situation where
we have two different function orders.

So let's rewrite the SA initialization flow to have same order for
both in-kernel and user-space callers.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>