af_unix: preserve const qualifier in unix_sk()

We can change unix_sk() to propagate its argument const qualifier,
thanks to container_of_const().

We need to change dump_common_audit_data() 'struct unix_sock *u'
local var to get a const attribute.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>