net - Add AF_ALG macros

This patch adds the socket family/level macros for the yet-to-be-born
AF_ALG family.  The AF_ALG family provides the user-space interface
for the kernel crypto API.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: David S. Miller <davem@davemloft.net>