lib: add tst_kvercmp2 to compare specific vendor extra kernel versions

We can now test specific verdor kernel versions in
test cases like:

static struct tst_kern_exv exv[] = {
	{"RHEL6", "234"},
	{"SLES10", "0.91"},
	{NULL, NULL}
};

...

	if (tst_kvercmp2(2, 6, 25, exv) < 0)
		...
...

Thank you for Cyril's suggestion.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Caspar Zhang <caspar@casparzhang.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>