block: fix aligning of bounced dio read bios

block: fix aligning of bounced dio read bios

bio_iov_iter_align_down expects the "normal" biovec layout from vector 0,
while bio_iov_iter_bounce_read abuses vector 0 for a bounce buffer
allocation.  Pass an explicit bvec to bio_iov_iter_align_down to deal
with this case to avoid a double unpin.

Additionally we need to free the folio if no bio_vec could be added,
and adjust the size of the first bio_vec that contains the bounce buffer
when the I/O size is aligned down.

Fixes: e7b8b3c5b2a6 ("block: align down bounces bios")
Reported-by: 0wnerD1ed <l7z@0b1t.tech>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: 0wnerD1ed <l7z@0b1t.tech>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://patch.msgid.link/20260716091306.316625-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>