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 uses union to set struct superblock size and uses
_Static_assert macro to check the size.

mkfs.c

@@ -11,8 +11,10 @@
 #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 +239,7 @@ static int write_data_blocks(int fd, struct superblock *sb)
 
 int main(int argc, char **argv)
 {
+    _Static_assert(sizeof(struct superblock) == SIMPLEFS_BLOCK_SIZE);
     if (argc != 2) {
         fprintf(stderr, "Usage: %s disk\n", argv[0]);
         return EXIT_FAILURE;