Squashed update of commits related to wait_for_completion_io ..
.. and writeback also

https://github.com/franciscofranco/hammerhead/commit/b34730c50e05a3c6a912bb342bb1a4b978a1f59a
https://github.com/franciscofranco/hammerhead/commit/9f3e4e611748ade2c4d4379cd830fbc1cd0604e3
https://github.com/franciscofranco/hammerhead/commit/723ceddd69a1d7bf6c8b50419a85d95e93d9f002
https://github.com/franciscofranco/hammerhead/commit/f5ce3093d9d8a3d75339771c73d2d54b37e0ff5a