ping6: Provide enough buffer for dn_comp() and make NI Query with Name subject work.

Because we did not supply enough buffer for dn_comp(), Subject Name
support for Node Information Query did not work.

I still do not understand why dn_comp() complains if we supplied
strlen(name) + 2...

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>