src/fiemap-tester: check for illegal block size

There was a regression in v4.19-rc1 that caused FIGETBSZ ioctl to return 0 on an overlayfs file.

That regression went unnoticed because the xfstests that run fiemap-tester program terminated in success status after not doing much instead of failing.

Check for invalid value of block size returned by FIGETBSZ ioctl, so these tests can detect the regression.

---
 src/fiemap-tester.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 97ac5ad0..5b0af478 100644
@@ -561,6 +561,12 @@ main(int argc, char **argv)
 		exit(1);
 	}
 
+	if (blocksize <= 0) {
+		printf("Illegal filesystem block size\n");
+		close(fd);
+		exit(1);
+	}
+
 #ifdef HAVE_FALLOCATE
 	/* if fallocate passes, then we can do preallocation, else not */
 	if (prealloc) {

2.17.1