lib/tst_kvercmp: Add support /etc/os-release

The kver on Ubuntu will be something like these:
* 4.4.0-187-generic
* 5.4.0-1021-kvm
* 4.15.0-1093-azure

So it's better to parse OS name from ID= in /etc/os-release, instead
of doing this from checking kver substring like what we did for RHEL
and Oracle Linux here.

From the document [1] this string will alway be in lowercase. Example:
"ID=fedora" or "ID=debian". Thus it needs to be converted to uppercase
to make it consistent with other return values in tst_kvcmp_distname().

[1] https://www.freedesktop.org/software/systemd/man/os-release.html

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>

Author: Cypresslin

lib/tst_kvercmp.c

@@ -18,13 +18,16 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <ctype.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <limits.h>
 #include <sys/utsname.h>
 #include "test.h"
 
+#define OSRELEASE_PATH "/etc/os-release"
+
 static char *parse_digit(const char *str, int *d)
 {
 	unsigned long v;
@@ -127,6 +130,9 @@ int tst_kvexcmp(const char *tst_exv, const char *cur_ver)
 
 const char *tst_kvcmp_distname(const char *kver)
 {
+	static char distname[64];
+	char *p = distname;
+
 	if (strstr(kver, ".el5uek"))
 		return "OL5UEK";
 
@@ -139,6 +145,17 @@ const char *tst_kvcmp_distname(const char *kver)
 	if (strstr(kver, ".el6"))
 		return "RHEL6";
 
+	if (access(OSRELEASE_PATH, F_OK) != -1) {
+		SAFE_FILE_LINES_SCANF(NULL, OSRELEASE_PATH, "ID=%s", distname);
+
+		while (*p) {
+			*p = toupper((unsigned char)*p);
+			p++;
+		}
+
+		return distname;
+	}
+
 	return NULL;
 }