Force spl.spl_hostid=0 when matching hostid

If the SPL kmod has a non-zero spl_hostid parameter, nothing we write to
/etc/hostid will have any effect. Forcing the module parameter to zero
will cause ZFS to look to /etc/hostid for a value regardless of the
initial state.

90zfsbootmenu/zfsbootmenu-lib.sh

@@ -122,7 +122,8 @@ center_string() {
 # returns: 0 on successful write, 1 on error
 
 write_hostid() {
-  local hostid ret
+  local hostid ret splmod
+  splmod="/sys/module/spl/parameters/spl_hostid"
 
   # Normalize the hostid
   if ! hostid="$( printf "%08x" "0x${1:-0}" 2>/dev/null )"; then
@@ -142,6 +143,10 @@ write_hostid() {
     ret=$?
   fi
 
+  if [ "${ret}" -eq 0 ] && [ -w "${splmod}" ]; then
+    echo 0 > "${splmod}" || zwarn "failed to force spl.spl_hostid=0 for host ID matching"
+  fi
+
   return ${ret}
 }