net/core: Collapse redundant sk_timer callback data assignments

The core sk_timer initializer can provide the common .data assignment
instead of it being set separately in users.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Andrew Hendry <andrew.hendry@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: linzhang <xiaolou4617@gmail.com>
Cc: netdev@vger.kernel.org
Cc: linux-hams@vger.kernel.org
Cc: linux-x25@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>