The commit 6a8d90f5fec4 "attr: Allow attribute type 0" intended to
allow the parsing of {netlink,packet,unix}_diag, even if they are
using type 0 for valid attributes.

It lacked this part in nla_parse.

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>