block: revert the iov_iter after a short copy in bio_iov_iter_bounce_write()

block: revert the iov_iter after a short copy in bio_iov_iter_bounce_write()

For the incoming IOMAP_DIO_BOUNCE flag usage inside btrfs, it's pretty
easy to hit short copy inside bio_iov_iter_bounce_write().

This is because btrfs has disabled page fault to avoid certain deadlock
during direct writes, and instead btrfs manually fault in the pages then
retry.

And inside bio_iov_iter_bounce_write(), if we hit a short write, we
didn't revert the iov_iter, which can cause problems like unexpected
garbage for the next retry.

Revert the iov_iter after a short copy.

One thing to note is that, the folio is allocated then immediately
queued into the bio, so the proper revert size should be
(bi_size - this_len + copied).

Fixes: 8dd5e7c75d7b ("block: add helpers to bounce buffer an iov_iter into bios")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/c400989f227343b134110773d5acaaacf7024574.1781597506.git.wqu@suse.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>