wireguard: socket: free skb in send6 when ipv6 is disabled

I got a memory leak report:

unreferenced object 0xffff8881191fc040 (size 232):
  comm "kworker/u17:0", pid 23193, jiffies 4295238848 (age 3464.870s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff814c3ef4>] slab_post_alloc_hook+0x84/0x3b0
    [<ffffffff814c8977>] kmem_cache_alloc_node+0x167/0x340
    [<ffffffff832974fb>] __alloc_skb+0x1db/0x200
    [<ffffffff82612b5d>] wg_socket_send_buffer_to_peer+0x3d/0xc0
    [<ffffffff8260e94a>] wg_packet_send_handshake_initiation+0xfa/0x110
    [<ffffffff8260ec81>] wg_packet_handshake_send_worker+0x21/0x30
    [<ffffffff8119c558>] process_one_work+0x2e8/0x770
    [<ffffffff8119ca2a>] worker_thread+0x4a/0x4b0
    [<ffffffff811a88e0>] kthread+0x120/0x160
    [<ffffffff8100242f>] ret_from_fork+0x1f/0x30

In function wg_socket_send_buffer_as_reply_to_skb() or wg_socket_send_
buffer_to_peer(), the semantics of send6() is required to free skb. But
when CONFIG_IPV6 is disable, kfree_skb() is missing. This patch adds it
to fix this bug.

Signed-off-by: Wang Hai <wanghai38@huawei.com>
Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>