tools/vm/slabinfo: indicates the cause of the EACCES error

If you don't run slabinfo with a superuser, return 0 when read_slab_dir()
reads get_obj_and_str("slabs", &t), because fopen() fails (sometimes
EACCES), causing slabcache() to return directly, without any error during
this time, we should tell the user about the EACCES problem instead of
running successfully($?=0) without any error printing.

For example:
$ ./slabinfo
Permission denied, Try using superuser <== What this submission did
$ sudo ./slabinfo
Name Objects Objsize Space Slabs/Part/Cpu O/S O %Fr %Ef Flg
Acpi-Namespace 5950 48 286.7K 65/0/5 85 0 0 99
Acpi-Operand 13664 72 999.4K 231/0/13 56 0 0 98
...

Signed-off-by: Rong Tao <[e-mail]>
Signed-off-by: Vlastimil Babka <[e-mail]>

tools/vm/slabinfo.c

···
 {
 	FILE *f = fopen(name, "r");
 
-	if (!f)
+	if (!f) {
 		buffer[0] = 0;
-	else {
+		if (errno == EACCES)
+			fatal("%s, Try using superuser\n", strerror(errno));
+	} else {
 		if (!fgets(buffer, sizeof(buffer), f))
 			buffer[0] = 0;
 		fclose(f);