xfrm: remove redundant parameter "int dir" in struct xfrm_mgr.acquire

Sematically speaking, xfrm_mgr.acquire is called when kernel intends to ask
user space IKE daemon to negotiate SAs with peers. IOW the direction will
*always* be XFRM_POLICY_OUT, so remove int dir for clarity.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>