Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Update posix_acl support to handle...

Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Update posix_acl support to handle user namespace mounts""

BugLink: https://bugs.launchpad.net/bugs/1644165
This reverts commit aad491810df581ee65a8415749850d555702bee7.

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>