wireguard: allowedips: avoid unaligned 64-bit memory accesses

On the parisc platform, the kernel issues kernel warnings because
swap_endian() tries to load a 128-bit IPv6 address from an unaligned
memory location:

 Kernel: unaligned access to 0x55f4688c in wg_allowedips_insert_v6+0x2c/0x80 [wireguard] (iir 0xf3010df)
 Kernel: unaligned access to 0x55f46884 in wg_allowedips_insert_v6+0x38/0x80 [wireguard] (iir 0xf2010dc)

Avoid such unaligned memory accesses by instead using the
get_unaligned_be64() helper macro.

Signed-off-by: Helge Deller <deller@gmx.de>
[Jason: replace src[8] in original patch with src+8]
Cc: stable@vger.kernel.org
Fixes: e7096c13

 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20240704154517.1572127-3-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>