The next inode number to be allocated has been stored only in the
in-memory super block and hasn't survived across mounts. This sometimes
accidentally worked if the tests removed the initial inodes but often
would cause failures when inode allocation returned existing inodes.
This tracks the next inode to allocate in the super block and maintains
it across mounts. Tests now consistently pass as inode allocations
consistently return free inode numbers.
Signed-off-by: Zach Brown <zab@versity.com>