mm/page_ext: move functions around for minor cleanups to page_ext

mm/page_ext: move functions around for minor cleanups to page_ext

1. move page_ext_get and page_ext_put down to remove forward
   declaration of lookup_page_ext.

2. move page_ext_init_flatmem_late down to existing non SPARS block to
   remove a new non SPARS block and to keep code for non SPARS tight.

Link: https://lkml.kernel.org/r/20230714114749.1743032-4-shikemeng@huaweicloud.com
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>