Actually, should I make it so that the bottom two bits of bi_flags are a
four-state variable and make it such that bio_release_page() gives a warning
if the state is 0 - ie. unset?

The states would then be, say:

	0	WARN(), do no cleanup
	1	FOLL_GET
	2	FOLL_PUT
	3	do no cleanup

This should help debug any places, such as iomap_dio_zero() that I just found,
that add pages with refs without calling iov_iter_extract_pages().

David