mm/filemap: Hoist filler_t decision to the top of do_read_cache_folio()

Now that filler_t and aops->read_folio() have the same type, we can decide
which one to use at the top of the function, and cache ->read_folio in
the filler parameter.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>