add support for ZFS datasets for systemd-fstab-generator

[Karl-Philipp Richter]

`legacy` mountpoints for ZFS datasets are specified as `[pool]/[dataset]` in `/etc/fstab` which `systemd-fstab-generator` doesn't understand and skips with warning `systemd-fstab-generator[20245]: Checking was requested for "data/data", but it is not a device.` which breaks dependency resolution for ZFS mountpoints.

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: systemd 231-9ubuntu1
ProcVersionSignature: Ubuntu 4.8.0-28.30-generic 4.8.6
Uname: Linux 4.8.0-28-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.3-0ubuntu8
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Dec 1 17:36:28 2016
InstallationDate: Installed on 2015-12-12 (355 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
MachineType: LENOVO 20221
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.8.0-28-generic root=UUID=791b47e7-915d-4d5a-bce0-d7e9b660a2ab ro rootflags=subvol=ubuntu-main-root
SourcePackage: systemd
UpgradeStatus: Upgraded to yakkety on 2016-10-17 (45 days ago)
dmi.bios.date: 07/12/2013
dmi.bios.vendor: LENOVO
dmi.bios.version: 71CN51WW(V1.21)
dmi.board.asset.tag: No Asset Tag
dmi.board.name: INVALID
dmi.board.vendor: LENOVO
dmi.board.version: 31900003WIN8 STD MLT
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Lenovo IdeaPad Z500 Touch
dmi.modalias: dmi:bvnLENOVO:bvr71CN51WW(V1.21):bd07/12/2013:svnLENOVO:pn20221:pvrLenovoIdeaPadZ500Touch:rvnLENOVO:rnINVALID:rvr31900003WIN8STDMLT:cvnLENOVO:ct10:cvrLenovoIdeaPadZ500Touch:
dmi.product.name: 20221
dmi.product.version: Lenovo IdeaPad Z500 Touch
dmi.sys.vendor: LENOVO

[Dan Streetman (ddstreet)]

please reopen if this is still an issue