loader: dosfs fails to access directory data with large cluster number

fsize is using 2 bytes for cluster number, but with fat32 we
actually do have 4 bytes and with large disks the high bytes will be in use.

illumos issue: https://www.illumos.org/issues/16821

Sponsored by:	MNX Cloud, Inc.

(cherry picked from commit 79a0d14)

stand/libsa/dosfs.c

@@ -816,7 +816,7 @@ fsize(DOS_FS *fs, DOS_DE *de)
 	int n;
 
 	if (!(size = cv4(de->size)) && de->attr & FA_DIR) {
-		if (!(c = cv2(de->clus))) {
+		if (!(c = stclus(fs->fatsz, de))) {
 			size = fs->dirents * sizeof(DOS_DE);
 		} else {
 			if ((n = fatcnt(fs, c)) == -1)