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: NMatthew Wilcox (Oracle) <willy@infradead.org>

mm/filemap.c

@@ -3487,6 +3487,9 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
 {
 	struct folio *folio;
 	int err;
+
+	if (!filler)
+		filler = mapping->a_ops->read_folio;
 repeat:
 	folio = filemap_get_folio(mapping, index);
 	if (!folio) {
@@ -3503,11 +3506,7 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
 		}
 
 filler:
-		if (filler)
-			err = filler(file, folio);
-		else
-			err = mapping->a_ops->read_folio(file, folio);
-
+		err = filler(file, folio);
 		if (err < 0) {
 			folio_put(folio);
 			return ERR_PTR(err);