ipv6: Add ipv6_addr_{cpu_to_be32,be32_to_cpu} helpers

Add helpers to convert an ipv6 addr, expressed as an array
of words, from CPU to big-endian byte order, and vice versa.

No functional change intended.
Compile tested only.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/netdev/c7684349-535c-45a4-9a74-d47479a50020@lunn.ch/


Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240813-ipv6_addr-helpers-v2-1-5c974f8cca3e@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>