Ensure superblock size always SIMPLEFS_BLOCK_SIZE

In current implementation, the struct superblock used padding via
subtraction to match the SIMPLEFS_BLOCK_SIZE, which could lead
to the need for recalculating the padding when the data structure
changed.

This commit uses union to set struct superblock size and uses
assert macro to check the size.

mkfs.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include <fcntl.h>
 #include <linux/fs.h>
 #include <stdint.h>
@@ -11,8 +12,10 @@
 #include "simplefs.h"
 
 struct superblock {
-    struct simplefs_sb_info info;
-    char padding[4064]; /* Padding to match block size */
+    union {
+        struct simplefs_sb_info info;
+        char padding[SIMPLEFS_BLOCK_SIZE]; /* Padding to match block size */
+    };
 };
 
 /**
@@ -237,6 +240,7 @@ static int write_data_blocks(int fd, struct superblock *sb)
 
 int main(int argc, char **argv)
 {
+    assert(sizeof(struct superblock) == SIMPLEFS_BLOCK_SIZE);
     if (argc != 2) {
         fprintf(stderr, "Usage: %s disk\n", argv[0]);
         return EXIT_FAILURE;