Add templated zfs-mount@.service

Runs `zfs mount -R <dataset>` at boot, after `zfs mount -a`.
Intended to replace `mountpoint=legacy` in certain mount setups.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Meriel Luna Mittelbach <lunarlambda@gmail.com>
Closes #17483

etc/Makefile.am

@@ -56,6 +56,7 @@ systemdunit_DATA = \
 	%D%/systemd/system/zfs-import-scan.service \
 	%D%/systemd/system/zfs-import.target \
 	%D%/systemd/system/zfs-mount.service \
+	%D%/systemd/system/zfs-mount@.service \
 	%D%/systemd/system/zfs-scrub-monthly@.timer \
 	%D%/systemd/system/zfs-scrub-weekly@.timer \
 	%D%/systemd/system/zfs-scrub@.service \

etc/systemd/system/zfs-mount@.service.in

@@ -0,0 +1,26 @@
+[Unit]
+Description=Mount ZFS filesystem %I
+Documentation=man:zfs(8)
+DefaultDependencies=no
+After=systemd-udev-settle.service
+After=zfs-import.target
+After=zfs-mount.service
+After=systemd-remount-fs.service
+Before=local-fs.target
+ConditionPathIsDirectory=/sys/module/zfs
+
+# This merely tells the service manager
+# that unmounting everything undoes the
+# effect of this service. No extra logic
+# is ran as a result of these settings.
+Conflicts=umount.target
+Before=umount.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+EnvironmentFile=-@initconfdir@/zfs
+ExecStart=@sbindir@/zfs mount -R %I
+
+[Install]
+WantedBy=zfs.target