tools: hv: Netlink source address validation allows DoS

The source code without this patch caused hypervkvpd to exit when it processed
a spoofed Netlink packet which has been sent from an untrusted local user.
Now Netlink messages with a non-zero nl_pid source address are ignored
and a warning is printed into the syslog.

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Acked-by:  K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static int kvp_write_file(FILE *f, char *s1, char *s2, char *s3)
{
	int ret;

	ret = fprintf(f, "%s%s%s%s\n", s1, s2, "=", s3);

	if (ret < 0)
		return HV_E_FAIL;

	return 0;
}

static int kvp_write_file(FILE *f, char *s1, char *s2, char *s3)
{
	int ret;

	ret = fprintf(f, "%s%s%s%s\n", s1, s2, "=", s3);

	if (ret < 0)
		return HV_E_FAIL;

	return 0;
}
