CVE-2024-42247 wireguard: allowedips: avoid unaligned 64-bit memory accesses

In the Linux kernel, the following vulnerability has been resolved:

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.

[Jason: replace src[8] in original patch with src+8]