net: clean up some sparse endianness warnings in ipv6.h

sparse is throwing warnings when building sunrpc modules due to some
endianness shenanigans in ipv6.h. Specifically:

  CHECK   net/sunrpc/addr.c
include/net/ipv6.h:573:17: warning: restricted __be64 degrades to integer
include/net/ipv6.h:577:34: warning: restricted __be32 degrades to integer
include/net/ipv6.h:573:17: warning: restricted __be64 degrades to integer
include/net/ipv6.h:577:34: warning: restricted __be32 degrades to integer

Sprinkle some endianness fixups to silence them. These should all get
fixed up at compile time, so I don't think this will add any extra work
to be done at runtime.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: David S. Miller <davem@davemloft.net>