Kernel Fixes to get TCMU File Backed Optical to work (V2)

[Tim Gardner]

https://bugs.launchpad.net/bugs/1646204

v2 - corrected patch 2/4 backport according to comments made in
     https://lists.ubuntu.com/archives/kernel-team/2016-December/081290.html

[PATCH 1/4] target/user: Use sense_reason_t in tcmu_queue_cmd_ring
[PATCH 2/4] target/user: Return an error if cmd data size is too
[PATCH 3/4] target/user: Fix comments to not refer to data ring
[PATCH 4/4] UBUNTU: SAUCE: (no-up) target/user: Fix use-after-free of

rtg