Changed fsinfo.minor_version -> fsinfo.disk_version

Version are now returned with major/minor packed into 32-bits,
so 0x00020001 is the current disk version, for example.

1. This needed to change to use a disk_* prefix for consistency with the
   defines that already exist for LFS_VERSION/LFS_DISK_VERSION.

2. Encoding the version this way has the nice side effect of making 0 an
   invalid value. This is useful for adding a similar config option
   that needs to have reasonable default behavior for backwards
   compatibility.

In theory this uses more space, but in practice most other config/status
is 32-bits in littlefs. We would be wasting this space for alignment
anyways.

lfs.h

@@ -282,8 +282,8 @@ struct lfs_info {
 
 // Filesystem info structure
 struct lfs_fsinfo {
-    // On-disk minor version.
-    uint16_t minor_version;
+    // On-disk version.
+    uint32_t disk_version;
 
     // Number of blocks in use, this is the same as lfs_fs_size.
     //