MBL: Add support for max z endstop

See https://github.com/MarlinFirmware/Marlin/issues/4088

Marlin/Marlin_main.cpp

   #if ENABLED(MESH_BED_LEVELING)
     if (mbl.has_mesh()) {
       if (home_all_axis || (axis_homed[X_AXIS] && axis_homed[Y_AXIS] && homeZ)) {
-        current_position[Z_AXIS] = MESH_HOME_SEARCH_Z;
+        current_position[Z_AXIS] = MESH_HOME_SEARCH_Z
+          #if Z_HOME_DIR > 0
+            + Z_MAX_POS
+          #endif
+        ;
         SYNC_PLAN_POSITION_KINEMATIC();
         mbl.set_active(true);
         #if ENABLED(MESH_G28_REST_ORIGIN)
         #else
           current_position[Z_AXIS] = MESH_HOME_SEARCH_Z -
             mbl.get_z(current_position[X_AXIS] - home_offset[X_AXIS],
-                      current_position[Y_AXIS] - home_offset[Y_AXIS]);
+                      current_position[Y_AXIS] - home_offset[Y_AXIS])
+            #if Z_HOME_DIR > 0
+              + Z_MAX_POS
+            #endif
+          ;
         #endif
       }
       else if ((axis_homed[X_AXIS] && axis_homed[Y_AXIS] && axis_homed[Z_AXIS]) && (homeX || homeY)) {
         // For each G29 S2...
         if (probe_point == 0) {
           // For the intial G29 S2 make Z a positive value (e.g., 4.0)
-          current_position[Z_AXIS] = MESH_HOME_SEARCH_Z;
+          current_position[Z_AXIS] = MESH_HOME_SEARCH_Z
+            #if Z_HOME_DIR > 0
+              + Z_MAX_POS
+            #endif
+          ;
           SYNC_PLAN_POSITION_KINEMATIC();
         }
         else {

Marlin/ultralcd.cpp

       if (lcdDrawUpdate) lcd_implementation_drawedit(PSTR(MSG_LEVEL_BED_WAITING));
       if (LCD_CLICKED) {
         _lcd_level_bed_position = 0;
-        current_position[Z_AXIS] = MESH_HOME_SEARCH_Z;
+        current_position[Z_AXIS] = MESH_HOME_SEARCH_Z
+          #if Z_HOME_DIR > 0
+            + Z_MAX_POS
+          #endif
+        ;
         planner.set_position_mm(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS], current_position[E_AXIS]);
         lcd_goto_screen(_lcd_level_goto_next_point, true);
       }