block: Fix iterating over an empty bio with bio_for_each_folio_all

In the Linux kernel, the following vulnerability has been resolved: block: Fix iterating over an empty bio with bio_for_each_folio_all If the bio contains no data, bio_first_folio() calls page_folio() on a NULL pointer and oopses. Move the test that we've reached the end of the bio from bio_next_folio() to bio_first_folio(). [axboe: add unlikely() to error case]