Read block size and check magic number from superblock
Add in-memory structure for file system
Bug fix: clean unmount

- iput() in put_super() freezes the system so remove it
- add invalidate_bdev() in put_super()
- if inode is not new then return it in inode_get()
Read inode from disk

- Calculate the block number of inode
- Read inode block from disk
- Format command inits the inode table as per inode structure
Merge pull request #4 from octabrain/master

Read file system information from superblock