Ensure superblock always fits 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 introduces a union that allocates the largest size among
its members to set the struct superblock size, ensuring that it is at
least SIMPLEFS_BLOCK_SIZE. Additionally, it utilizes the
_Static_assert keyword to verify whether the size matches
SIMPLEFS_BLOCK_SIZE.

mkfs.c

@@ -11,10 +11,14 @@
 #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 */
+    };
 };
 
+_Static_assert(sizeof(struct superblock) == SIMPLEFS_BLOCK_SIZE);
+
 /**
  * DIV_ROUND_UP - round up a division
  * @n: dividend