crypto: af_alg - Avoid sock_graft call warning

The newly added sock_graft warning triggers in af_alg_accept.
It's harmless as we're essentially doing sock->sk = sock->sk.

The sock_graft call is actually redundant because all the work
it does is subsumed by sock_init_data.  However, it was added
to placate SELinux as it uses it to initialise its internal state.

This patch avoisd the warning by making the SELinux call directly.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: David S. Miller <davem@davemloft.net>