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>