logfs: destroy the reserved inodes while unmounting

We were assuming that the evict_inode() would never be called on
reserved inodes. However, (after the commit 8e22c1a4 logfs: get rid
of magical inodes) while unmounting the file system, in put_super, we
call iput() on all of the reserved inodes.

The following simple test used to cause a kernel panic on LogFS:

1. Mount a LogFS file system on /mnt

2. Create a file
   $ touch /mnt/a

3. Try to unmount the FS
   $ umount /mnt

The simple fix would be to drop the assumption and properly destroy
the reserved inodes.

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>