block: blk-mq: fix uninit-value in blk_rq_prep_clone and refactor

Fix an issue detected by the `smatch` tool:

block/blk-mq.c:3314 blk_rq_prep_clone() error: uninitialized
symbol 'bio'.

This patch refactors `blk_rq_prep_clone()` to improve code
readability and ensure safety by addressing potential misuse of
the `bio` variable:

- Move the bio_put(bio); call to the bio_ctr error handling block,
  which is the only place where it can be triggered.
- Move the bio variable into the __rq_for_each_bio loop scope.
  This change removes the need to set bio to NULL at the loop's
  end.

discussion on why bio remains uninitialized:
https://lore.kernel.org/lkml/20241004141037.43277-1-surajsonawane0215@gmail.com

Summary of above discussion:
- I pointed out that `bio` can remain uninitialized if the
  allocation with `bio_alloc_clone` fails.
- Keith Busch explained that `bio` is initialized to `NULL` when
  `bio_alloc_clone()` fails, preventing uninitialized usage.
- John Garry questioned whether `rq_src->bio` being `NULL` could
  leave `bio` uninitialized. Keith clarified that in such cases,
  `bio` is not referenced, so it does not need initialization.
- Christoph Hellwig recommended code improvements:
 - move the bio_put to the bio_ctr error handling, which is the only
   case where it can happen
 - move the bio variable into the __rq_for_each_bio scope, which
   also removed the need to zero it at the end of the loop

These changes enhance code clarity, address static analysis tool
warnings, and make the function more maintainable.

thread of previous version patch discussion:
https://lore.kernel.org/lkml/20241004100842.9052-1-surajsonawane0215@gmail.com

Signed-off-by: Suraj Sonawane <surajsonawane0215@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241119164412.37609-1-surajsonawane0215@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: SurajSonawane2415

block/blk-mq.c

@@ -3273,27 +3273,28 @@ int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
 		      int (*bio_ctr)(struct bio *, struct bio *, void *),
 		      void *data)
 {
-	struct bio *bio, *bio_src;
+	struct bio *bio_src;
 
 	if (!bs)
 		bs = &fs_bio_set;
 
 	__rq_for_each_bio(bio_src, rq_src) {
-		bio = bio_alloc_clone(rq->q->disk->part0, bio_src, gfp_mask,
-				      bs);
+		struct bio *bio	 = bio_alloc_clone(rq->q->disk->part0, bio_src,
+					gfp_mask, bs);
 		if (!bio)
 			goto free_and_out;
 
-		if (bio_ctr && bio_ctr(bio, bio_src, data))
+		if (bio_ctr && bio_ctr(bio, bio_src, data)) {
+			bio_put(bio);
 			goto free_and_out;
+		}
 
 		if (rq->bio) {
 			rq->biotail->bi_next = bio;
 			rq->biotail = bio;
 		} else {
 			rq->bio = rq->biotail = bio;
 		}
-		bio = NULL;
 	}
 
 	/* Copy attributes of the original request to the clone request. */
@@ -3311,8 +3312,6 @@ int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
 	return 0;
 
 free_and_out:
-	if (bio)
-		bio_put(bio);
 	blk_rq_unprep_clone(rq);
 
 	return -ENOMEM;