kconfig: gconf: Fix Back button behavior

Clicking the Back button may navigate to a non-menu hierarchy level.

[Example]

  menu "menu1"

  config A
          bool "A"
          default y

  config B
          bool "B"
          depends on A
          default y

  menu "menu2"
          depends on B

  config C
          bool "C"
          default y

  endmenu

  endmenu

After being re-parented by menu_finalize(), the menu tree is structured
like follows:

  menu "menu1"
  \-- A
      \-- B
          \-- menu "menu2"
              \-- C

In Single view, visit "menu2" and click the Back button. It should go up
to "menu1" and show A, B and "menu2", but instead goes up to A and show
only B and "menu2". This is a bug in on_back_clicked().

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>