netlink: don't compare the nul-termination in nla_strcmp

[ Upstream commit 8b7b932434f5eee495b91a2804f5b64ebb2bc835 ]

nla_strcmp compares the string length plus one, so it's implicitly
including the nul-termination in the comparison.

 int nla_strcmp(const struct nlattr *nla, const char *str)
 {
        int len = strlen(str) + 1;
        ...
                d = memcmp(nla_data(nla), str, len);

However, if NLA_STRING is used, userspace can send us a string without
the nul-termination. This is a problem since the string
comparison will not match as the last byte may be not the
nul-termination.

Fix this by skipping the comparison of the nul-termination if the
attribute data is nul-terminated. Suggested by Thomas Graf.

Change-Id: I880183f7bf15d1c7aee9acc682f393744ffd399a
Cc: Florian Westphal <fw@strlen.de>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>