ipv6: Use __fls() instead of fls() in __ipv6_addr_diff().

Because we have ensured that the argument is non-zero,
it is better to use __fls() and generate better code.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>