mm/truncate.c: make __invalidate_mapping_pages() static

Fix the following sparse warning:

  mm/truncate.c:531:15: warning: symbol '__invalidate_mapping_pages' was not declared. Should it be static?

Fixes: eb1d7a65 ("mm, fadvise: improve the expensive remote LRU cache draining after FADV_DONTNEED")
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lkml.kernel.org/r/20201015054808.2445904-1-yanaijie@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>