Build ZFS directly from upstream

This change updates the build process to directly build the kernel from
upstream sources, as opposed to requiring the Proxmox zfsonlinux
repository to build the patched sources.

debian/patches/zfs/0008-Add-systemd-unit-for-importing-specific-pools.patch

@@ -0,0 +1,66 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Stoiko Ivanov <s.ivanov@proxmox.com>
+Date: Tue, 15 Sep 2020 19:07:24 +0200
+Subject: [PATCH] Add systemd-unit for importing specific pools
+
+The unit can be instantiated with a specific poolname, which will get imported
+by scanning /dev/disk/by-id, irrespective of the existence and content of
+/etc/zfs/zpool.cache.
+
+the instance name is used unescaped (see systemd.unit(5)), since zpool names
+can contain characters which will be escaped by systemd.
+
+Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
+---
+ etc/systemd/system/50-zfs.preset.in       |  1 +
+ etc/systemd/system/Makefile.am            |  1 +
+ etc/systemd/system/zfs-import@.service.in | 16 ++++++++++++++++
+ 3 files changed, 18 insertions(+)
+ create mode 100644 etc/systemd/system/zfs-import@.service.in
+
+diff --git a/etc/systemd/system/50-zfs.preset.in b/etc/systemd/system/50-zfs.preset.in
+index e4056a92c..030611419 100644
+--- a/etc/systemd/system/50-zfs.preset.in
++++ b/etc/systemd/system/50-zfs.preset.in
+@@ -1,6 +1,7 @@
+ # ZFS is enabled by default
+ enable zfs-import-cache.service
+ disable zfs-import-scan.service
++enable zfs-import@.service
+ enable zfs-import.target
+ enable zfs-mount.service
+ enable zfs-share.service
+diff --git a/etc/systemd/system/Makefile.am b/etc/systemd/system/Makefile.am
+index c374a52ac..25d1b99d7 100644
+--- a/etc/systemd/system/Makefile.am
++++ b/etc/systemd/system/Makefile.am
+@@ -7,6 +7,7 @@ systemdunit_DATA = \
+ 	zfs-zed.service \
+ 	zfs-import-cache.service \
+ 	zfs-import-scan.service \
++	zfs-import@.service \
+ 	zfs-mount.service \
+ 	zfs-share.service \
+ 	zfs-volume-wait.service \
+diff --git a/etc/systemd/system/zfs-import@.service.in b/etc/systemd/system/zfs-import@.service.in
+new file mode 100644
+index 000000000..9b4ee9371
+--- /dev/null
++++ b/etc/systemd/system/zfs-import@.service.in
+@@ -0,0 +1,16 @@
++[Unit]
++Description=Import ZFS pool %i
++Documentation=man:zpool(8)
++DefaultDependencies=no
++After=systemd-udev-settle.service
++After=cryptsetup.target
++After=multipathd.target
++Before=zfs-import.target
++
++[Service]
++Type=oneshot
++RemainAfterExit=yes
++ExecStart=@sbindir@/zpool import -N -d /dev/disk/by-id -o cachefile=none %I
++
++[Install]
++WantedBy=zfs-import.target