xfrm: Fix acquire state insertion.

A recent commit jumped over the dst hash computation and
left the symbol uninitialized. Fix this by explicitly
computing the dst hash before it is used.

Fixes: 0045e3d8 ("xfrm: Cache used outbound xfrm states at the policy.")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>