FROMLIST: mm: don't cast ->readpage to filler_t for do_read_cache_page

We can just pass a NULL filler and do the right thing inside of
do_read_cache_page based on the NULL parameter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Bug: 133186739
Change-Id: I720afb2e0c28d5649b62ed3852b48ac63dc0d7a8
Link: https://lkml.org/lkml/2019/5/1/294


Signed-off-by: Sami Tolvanen <samitolvanen@google.com>