mkfat: zero out disk as part of format

Signed-off-by: Sean Cross <sean@xobs.io>

src/mkfat.c

@@ -159,5 +159,13 @@ int fat12_mkfs(struct pang_io *io, uint32_t bytes) {
         offset += sector_size;
     }
 
+    // Fill in the rest of the disk
+    while (offset < bytes) {
+        if (pang_write(io, offset, mbr, sizeof(mbr)) != sizeof(mbr)) {
+            return -1;
+        }
+        offset += sector_size;
+    }
+
     return 0;
 }