config/dracut/90zfs: handle cases where hostid(1) returns all zeros

On systems with musl libc, hostid(1) always prints "00000000", which
will cause improper behavior when the 90zfs module is configured in a
dracut initramfs. Work around this by copying the host /etc/hostid if
the file exists, and otherwise only write /etc/hostid if hostid(1)
returns something meaningful. This avoids zgenhostid creating a random
/etc/hostid for the initramfs, which could lead to errors when trying to
import the pool if spl_hostid isn't defined in the kernel command line.

Furthermore, tag the /etc/hostid file as hostonly, since it is system
specific and shouldn't be taken into account when trying to use an
initramfs generated in one system to boot into a different system.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Georgy Yakovlev <[email protected]>
Co-authored-by: Andrew J. Hesford <[email protected]>
Signed-off-by: Érico Rolim <[email protected]>
Closes openzfs#11174
Closes openzfs#11189

contrib/dracut/90zfs/module-setup.sh.in

@@ -85,7 +85,13 @@ install() {
 	fi
 
 	# Synchronize initramfs and system hostid
-	zgenhostid -o "${initdir}/etc/hostid" "$(hostid)"
+	if [ -f @sysconfdir@/hostid ]; then
+		inst @sysconfdir@/hostid
+		type mark_hostonly >/dev/null 2>&1 && mark_hostonly @sysconfdir@/hostid
+	elif HOSTID="$(hostid 2>/dev/null)" && [ "${HOSTID}" != "00000000" ]; then
+		zgenhostid -o "${initdir}@sysconfdir@/hostid" "${HOSTID}"
+		type mark_hostonly >/dev/null 2>&1 && mark_hostonly @sysconfdir@/hostid
+	fi
 
 	if dracut_module_included "systemd"; then
 		mkdir -p "${initdir}/$systemdsystemunitdir/zfs-import.target.wants"