Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Allow superblock owner to change owner...

Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Allow superblock owner to change ownership of inodes with unmappable ids""

BugLink: https://bugs.launchpad.net/bugs/1644165
This reverts commit 65d51ade0eb256f763adbd8b7a237c04056084e6.

The kernel fix for bug #1634964 breaks LXD userspace, in particular the
following commits:

ac7f3f73cb39 (namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs
ca52383ad6a6 (namespace) vfs: Don't create inodes with a uid or gid unknown to the vfs

LXD 2.0.6 will include changes to support these kernel changes, but it isn't
available yet on xenial, so for now we just revert these commits.

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>