bash completion of umount doesn't work for encfs mounts

[Karl-Philipp Richter]

`bash` completion with the Tab key doesn't work for `encfs` mounts after typing `sudo umount ` like it does for other mountpoints.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: bash-completion 1:2.1-4.3ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-19.22-generic 4.13.13
Uname: Linux 4.13.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.7-0ubuntu3.6
Architecture: amd64
CurrentDesktop: Unity:Unity7:ubuntu
Date: Thu Dec 14 15:32:55 2017
InstallationDate: Installed on 2015-12-12 (733 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: bash-completion
UpgradeStatus: Upgraded to artful on 2017-10-19 (55 days ago)