New service that waits on zvol links to be created

The zfs-volume-wait.service scans existing zvols and waits for their
links under /dev to be created. Any service that depends on zvol
links to be there should add a dependency on zfs-volumes.target.
By default, this target is not enabled.

Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Zakharov <pzakharov@delphix.com>
Closes #8975

etc/systemd/system/50-zfs.preset.in

@@ -5,4 +5,5 @@ enable zfs-import.target
 enable zfs-mount.service
 enable zfs-share.service
 enable zfs-zed.service
+enable zfs-volume-wait.service
 enable zfs.target

etc/systemd/system/Makefile.am

@@ -7,7 +7,9 @@ systemdunit_DATA = \
 	zfs-import-scan.service \
 	zfs-mount.service \
 	zfs-share.service \
+	zfs-volume-wait.service \
 	zfs-import.target \
+	zfs-volumes.target \
 	zfs.target
 
 EXTRA_DIST = \
@@ -17,6 +19,8 @@ EXTRA_DIST = \
 	$(top_srcdir)/etc/systemd/system/zfs-mount.service.in \
 	$(top_srcdir)/etc/systemd/system/zfs-share.service.in \
 	$(top_srcdir)/etc/systemd/system/zfs-import.target.in \
+	$(top_srcdir)/etc/systemd/system/zfs-volume-wait.service.in \
+	$(top_srcdir)/etc/systemd/system/zfs-volumes.target.in \
 	$(top_srcdir)/etc/systemd/system/zfs.target.in \
 	$(top_srcdir)/etc/systemd/system/50-zfs.preset.in
 

etc/systemd/system/zfs-volume-wait.service.in

@@ -0,0 +1,13 @@
+[Unit]
+Description=Wait for ZFS Volume (zvol) links in /dev
+DefaultDependencies=no
+After=systemd-udev-settle.service
+After=zfs-import.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=@bindir@/zvol_wait
+
+[Install]
+WantedBy=zfs-volumes.target

etc/systemd/system/zfs-volumes.target.in

@@ -0,0 +1,7 @@
+[Unit]
+Description=ZFS volumes are ready
+After=zfs-volume-wait.service
+Requires=zfs-volume-wait.service
+
+[Install]
+WantedBy=zfs.target