Inode state
Inode can be on one or two lists:
- (hash & in_use) or (hash & dirty ) or unused
- inode has a use count i_count
Transitions
- unused ? hash: iget calls sss_read_inode
- dirty? in_use: sss_write_inode
- hash ? unused: call on sss_clear_inode, but if
i_nlink = 0: iput calls sss_delete_inode when i_count falls to 0