scoutfs: initialize block alloc past mkfs blocks

The format doesn't yet record free blocks.  We've been relying on the
scary initialization of the block allocator past the blocks that are
written by mkfs.  And it was wrong.  This garbage will be replaced with
an allocator in a few commits.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/super.c

@@ -80,7 +80,7 @@ static int read_supers(struct super_block *sb)
 	 * XXX These don't exist in the super yet.  They should soon.
 	 */
 	atomic64_set(&sbi->next_ino, SCOUTFS_ROOT_INO + 1);
-	atomic64_set(&sbi->next_blkno, 2);
+	atomic64_set(&sbi->next_blkno, 6);
 
 	return 0;
 }