lib/xe/xe_gt: Repair xe_gt_stats_get_count function

Resolve the following issues:

- The target directory for the fopen does not take into consideration
  the dri subdirectory and assumes the correct path is dri/0.  This
  might not be the case.  Use the "minor" function and fstat to
  generate the correct subdirectory.

- The fopen may fail and return a NULL pointer, causing the subsequent
  fgets to generate a SIGSEGV.  Assert the file opened correctly.

- The file opened by fopen is never closed.  Close it.

v2: fclose (Kamil)

Fixes: 10cefd82 ("tests/intel/xe_tlb: Add test to check TLB invalidation")
Closes: drm/xe/kernel#3212


Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Suggested-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
CC: Sai Gowtham Ch <sai.gowtham.ch@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>

lib/xe/xe_gt.c

@@ -152,18 +152,28 @@ void xe_post_hang_ring(int fd, igt_hang_t arg)
 int xe_gt_stats_get_count(int fd, int gt, const char *stat)
 {
 	FILE *f;
+	struct stat st;
 	char tlb_path[4096];
 	char path[256];
 	int count;
 
-	sprintf(path, "/sys/kernel/debug/dri/0/gt%d/stats", gt);
+	igt_assert_eq(fstat(fd, &st), 0);
+
+	sprintf(path, "/sys/kernel/debug/dri/%d/gt%d/stats",
+		minor(st.st_rdev), gt);
 	f = fopen(path, "r");
 
+	igt_assert_f(f, "Failed to open /sys/kernel/debug/dri/%d/gt%d/stats",
+		     minor(st.st_rdev), gt);
+
 	while (fgets(tlb_path, sizeof(tlb_path), f)) {
 		if (strstr(tlb_path, stat) != NULL) {
 			sscanf(tlb_path, "%*[^:]: %d", &count);
 			break;
 		}
 	}
+
+	fclose(f);
+
 	return count;
 }