[fs] VFS Superblock

[ppenna]

Description

We should provide a superblock abstraction for the VFS. Overall, the following interface should be provided:

• [ ] superblock_init(): initializes a superblock
• [ ] superblock_lock(): locks a superblock
• [ ] superblock_unlock(): unlocks a superblock
• [ ] superblock_get(): gets a reference to a superblock
• [ ] superblock_put(): releases the reference a superblock
• [ ] superblock_read(): reads a superblock
• [ ] superblock_stat(): gets information of a superblock
• [ ] superblock_sync(): synchronizes a superblock with the back storage