Update fuse to translate uids and gids to/from the user namspace
of the process servicing requests on /dev/fuse. Any ids which do
not map into the namespace will result in errors. inodes will
also be marked bad when unmappable ids are received from the
userspace fuse process.

Currently no use cases are known for letting the userspace fuse
daemon switch namespaces after opening /dev/fuse. Given this
fact, and in order to keep the implementation as simple as
possible and ease security auditing, the user namespace from
which /dev/fuse is opened is used for all id translations. This
is required to be the same namespace as s_user_ns to maintain
behavior consistent with other filesystems which can be mounted
in user namespaces.

For cuse the namespace used for the connection is also simply
current_user_ns() at the time /dev/cuse is opened.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>