mm: use filemap_get_entry in filemap_get_incore_folio

filemap_get_incore_folio wants to look at the details of xa_is_value
entries, but doesn't need any of the other logic in filemap_get_folio. 
Switch it to use the lower-level filemap_get_entry interface.

Link: https://lkml.kernel.org/r/20230307143410.28031-4-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swap_state.c

@@ -386,7 +386,7 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping,
 {
 	swp_entry_t swp;
 	struct swap_info_struct *si;
-	struct folio *folio = __filemap_get_folio(mapping, index, FGP_ENTRY, 0);
+	struct folio *folio = filemap_get_entry(mapping, index);
 
 	if (!xa_is_value(folio))
 		goto out;