ipv6: exthdrs: use swap() instead of open coding it

Address the following coccicheck warning:
net/ipv6/exthdrs.c:620:44-45: WARNING opportunity for swap()

by using swap() for the swapping of variable values and drop
the tmp (`addr`) variable that is not needed any more.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>