xfrm: constify mark argument of xfrm_find_acq()

The mark argument is read only, so constify it. Also make dummy_mark in
af_key const -- only used as dummy argument for this very function.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/key/af_key.c

@@ -45,7 +45,7 @@ struct netns_pfkey {
 static DEFINE_MUTEX(pfkey_mutex);
 
 #define DUMMY_MARK 0
-static struct xfrm_mark dummy_mark = {0, 0};
+static const struct xfrm_mark dummy_mark = {0, 0};
 struct pfkey_sock {
 	/* struct sock must be the first member of struct pfkey_sock */
 	struct sock	sk;