crypto: user - constify netlink dispatch table

There is no need to modify the netlink dispatch table at runtime and
making it const even makes the resulting object file slightly smaller.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>