mm/PAGE_IDLE_FLAG: Set PAGE_EXTENSION for none-64BIT

mm/PAGE_IDLE_FLAG: Set PAGE_EXTENSION for none-64BIT

Commit 128fd80c4c07 ("mm/idle_page_tracking: Make PG_idle reusable") of
linux-mm[1] allows PAGE_IDLE_FLAG to be set without PAGE_EXTENSION while
64BIT is not set.  This makes 'enum page_ext_flags' undefined, so build
fails as below for the config (!64BIT, !PAGE_EXTENSION, and
IDLE_PAGE_FLAG).

    $ make ARCH=i386 allnoconfig
    $ echo 'CONFIG_PAGE_IDLE_FLAG=y' >> .config
    $ make olddefconfig
    $ make ARCH=i386
    [...]
    ../include/linux/page_idle.h: In function `folio_test_young':
    ../include/linux/page_idle.h:25:18: error: `PAGE_EXT_YOUNG' undeclared (first use in this function); did you mean `PAGEOUTRUN'?
       return test_bit(PAGE_EXT_YOUNG, &page_ext->flags);
    [...]

This commit fixes this issue by making PAGE_EXTENSION to be set when 64BIT
is not set and PAGE_IDLE_FLAG is set.

[1] https://github.com/hnaz/linux-mm/commit/128fd80c4c07

Link: https://lkml.kernel.org/r/20210806095153.6444-1-sj38.park@gmail.com
Fixes: 128fd80c4c07 ("mm/idle_page_tracking: Make PG_idle reusable")
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>