Does the patch below fix the warning for you?

--
From 28aae7104425433d39e6142adcd5b88dc5b0ad5f Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Thu, 5 Oct 2017 18:31:02 +0200
Subject: block: use DECLARE_COMPLETION_ONSTACK in submit_bio_wait

This way we get our lockdep annotations right in case multiple layers
in the stack use submit_bio_wait.

It also happens to simplify the code by getting rid of the submit_bio_ret
structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/bio.c | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 8338304ea256..4e18e959fc0a 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -917,17 +917,9 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 }
 EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);

-struct submit_bio_ret {
-       struct completion event;
-       int error;
-};
-
 static void submit_bio_wait_endio(struct bio *bio)
 {
-       struct submit_bio_ret *ret = bio->bi_private;
-
-       ret->error = blk_status_to_errno(bio->bi_status);
-       complete(&ret->event);
+       complete(bio->bi_private);
 }

 /**

@@ -943,16 +935,15 @@ static void submit_bio_wait_endio(struct bio *bio)
  */
 int submit_bio_wait(struct bio *bio)
 {
-       struct submit_bio_ret ret;
+       DECLARE_COMPLETION_ONSTACK(done);

-       init_completion(&ret.event);
-       bio->bi_private = &ret;
+       bio->bi_private = &done;
        bio->bi_end_io = submit_bio_wait_endio;
        bio->bi_opf |= REQ_SYNC;
        submit_bio(bio);
-       wait_for_completion_io(&ret.event);
+       wait_for_completion_io(&done);

-       return ret.error;
+       return blk_status_to_errno(bio->bi_status);
 }
 EXPORT_SYMBOL(submit_bio_wait);