LoongArch: Fix build errors due to backported TIMENS

Commit eb3710ef ("LoongArch: Add support
to clone a time namespace") backports the TIMENS support for LoongArch
(corresponding upstream commit aa5e65dc)
but causes build errors:

  CC      arch/loongarch/kernel/vdso.o
arch/loongarch/kernel/vdso.c: In function ‘vvar_fault’:
arch/loongarch/kernel/vdso.c:54:36: error: implicit declaration of
function ‘find_timens_vvar_page’ [-Werror=implicit-function-declaration]
   54 |         struct page *timens_page = find_timens_vvar_page(vma);
      |                                    ^~~~~~~~~~~~~~~~~~~~~
arch/loongarch/kernel/vdso.c:54:36: warning: initialization of ‘struct
page *’ from ‘int’ makes pointer from integer without a cast
[-Wint-conversion]
arch/loongarch/kernel/vdso.c: In function ‘vdso_join_timens’:
arch/loongarch/kernel/vdso.c:143:25: error: implicit declaration of
function ‘zap_vma_pages’; did you mean ‘zap_vma_ptes’?
[-Werror=implicit-function-declaration]
  143 |                         zap_vma_pages(vma);
      |                         ^~~~~~~~~~~~~
      |                         zap_vma_ptes
cc1: some warnings being treated as errors

Because in 6.1.y we should define find_timens_vvar_page() by ourselves
and use zap_page_range() instead of zap_vma_pages(), so fix it.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>