Add nfs-kernel-server for Debian

Debian based systems use nfs-kernel-server as the service name.
List both nfs-server.service and nfs-kernel-server.service so
this service will work on multiple distributions.

Signed-off-by: Grischa Zengel <github.zfsonlinux@zengel.info>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4350

etc/systemd/system/zfs-share.service.in

@@ -1,10 +1,10 @@
 [Unit]
 Description=ZFS file system shares
-After=nfs-server.service
+After=nfs-server.service nfs-kernel-server.service
 After=smb.service
 After=zfs-mount.service
 Requires=zfs-mount.service
-PartOf=nfs-server.service
+PartOf=nfs-server.service nfs-kernel-server.service
 PartOf=smb.service
 
 [Service]