fs: dlm: use sk->sk_socket instead of con->sock

Instead of dereference "con->sock" we can get the socket structure over
"sk->sk_socket" as well. This patch will switch to this behaviour.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>