ANDROID: mm: page_pinner: fix build warning

Fix build warning below.

> All warnings (new ones prefixed by >>):
>
> >> mm/page_pinner.c:304:22: warning: variable 'page_pinner' set but not used [-Wunused-but-set-variable]
>            struct page_pinner *page_pinner;
>                                ^
>    1 warning generated.
>

Fixes: ddc4a487 ("ANDROID: mm: page_pinner: introduce failure_tracking feature")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I02cfd98f9c33745920f883d5e2e2bc2730f662a2