Revert "(namespace) Revert "UBUNTU: SAUCE: ima/evm: Allow root in s_user_ns to set...

Revert "(namespace) Revert "UBUNTU: SAUCE: ima/evm: Allow root in s_user_ns to set xattrs""

BugLink: https://bugs.launchpad.net/bugs/1644165
This reverts commit db19ff87e3f993e46217f369a066f9d1d4691df8.

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>