ipv6: exthdrs: use swap macro in ipv6_dest_hao

make use of the swap macro and remove unnecessary variable tmp_addr.
This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>