Revert "(namespace) fs: Refuse uid/gid changes which don't map into s_user_ns"

Revert "(namespace) fs: Refuse uid/gid changes which don't map into s_user_ns"

BugLink: https://bugs.launchpad.net/bugs/1644165
This reverts commit dc0d66cb531fc0a98c01e292816fb08f16916af8.

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>