romfs: fix romfs_read_folio()

Add the correct offset to folio_zero_tail().

Fixes: d86f2de0 ("romfs: Convert romfs_read_folio() to use a folio")
Reported-by: Greg Ungerer <gregungerer@westnet.com.au>
Link: https://lore.kernel.org/r/Zr0GTnPHfeA0P8nb@casper.infradead.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/romfs/super.c

@@ -126,7 +126,7 @@ static int romfs_read_folio(struct file *file, struct folio *folio)
 		}
 	}
 
-	buf = folio_zero_tail(folio, fillsize, buf);
+	buf = folio_zero_tail(folio, fillsize, buf + fillsize);
 	kunmap_local(buf);
 	folio_end_read(folio, ret == 0);
 	return ret;