block: factor out a bio_await helper

block: factor out a bio_await helper

Add a new helper to wait for a bio and anything chained off it to
complete synchronously after submitting it.  This factors common code out
of submit_bio_wait and bio_await_chain and will also be useful for
file system code and thus is exported.

Note that this will now set REQ_SYNC also for the bio_await case for
consistency.  Nothing should look at the flag in the end_io handler,
but if something does having the flag set makes more sense.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://patch.msgid.link/20260407140538.633364-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>