Increased size of FAT filesystems by 1MB

This prevents a 'No space left on device' error from occuring
for large kernel images.

Implements suggestion in rust-osdev#438 by Ferdia McKeogh <[email protected]>

Author: MarkRoss470

src/fat.rs

@@ -26,7 +26,7 @@ pub fn create_fat_filesystem(
         .truncate(true)
         .open(out_fat_path)
         .unwrap();
-    let fat_size_padded_and_rounded = ((needed_size + 1024 * 64 - 1) / MB + 1) * MB + MB;
+    let fat_size_padded_and_rounded = ((needed_size + 1024 * 64 - 1) / MB + 2) * MB + MB;
     fat_file.set_len(fat_size_padded_and_rounded).unwrap();
 
     // choose a file system label