Do not set BIO_DONE if the BIO specifies a completion handler.

biowait() will otherwise race with completions of such BIOs. In-tree code
only calls biowait() on BIOs that do not specify a handler, so this change
should not have any functional impact.

Reviewed by:	mav
MFC after:	1 month
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D9070

sys/kern/vfs_bio.c

@@ -3905,11 +3905,9 @@ biodone(struct bio *bp)
 		bp->bio_flags |= BIO_DONE;
 		wakeup(bp);
 		mtx_unlock(mtxp);
-	} else {
-		bp->bio_flags |= BIO_DONE;
+	} else
 		done(bp);
 }
-}
 
 /*
  * Wait for a BIO to finish.