block: fix segment calculation for passthrough IO

blk_recount_segments() can be called in bio_add_pc_page() for
calculating how many segments this bio will has after one page is added
to this bio. If the resulted segment number is beyond the queue limit,
the added page will be removed.

The try-and-fix policy requires blk_recount_segments(__blk_recalc_rq_segments)
to not consider the segment number limit. Unfortunately bvec_split_segs()
does check this limit, and causes small segment number returned to
bio_add_pc_page(), then page still may be added to the bio even though
segment number limit becomes broken.

Fixes this issue by not considering segment number limit when calcualting
bio's segment number.

Fixes: dcebd755926b ("block: use bio_for_each_bvec() to compute multi-page bvec count")
Cc: Christoph Hellwig <[e-mail]>
Cc: Omar Sandoval <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-merge.c

···
  */
 static bool bvec_split_segs(struct request_queue *q, struct bio_vec *bv,
 		unsigned *nsegs, unsigned *last_seg_size,
-		unsigned *front_seg_size, unsigned *sectors)
+		unsigned *front_seg_size, unsigned *sectors, unsigned max_segs)
 {
 	unsigned len = bv->bv_len;
 	unsigned total_len = 0;
···
 	 * Multi-page bvec may be too big to hold in one segment, so the
 	 * current bvec has to be splitted as multiple segments.
 	 */
-	while (len && new_nsegs + *nsegs < queue_max_segments(q)) {
+	while (len && new_nsegs + *nsegs < max_segs) {
 		seg_size = get_max_segment_size(q, bv->bv_offset + total_len);
 		seg_size = min(seg_size, len);
 
···
 	bool do_split = true;
 	struct bio *new = NULL;
 	const unsigned max_sectors = get_max_io_size(q, bio);
+	const unsigned max_segs = queue_max_segments(q);
 
 	bio_for_each_bvec(bv, bio, iter) {
 		/*
···
 			 * Consider this a new segment if we're splitting in
 			 * the middle of this vector.
 			 */
-			if (nsegs < queue_max_segments(q) &&
+			if (nsegs < max_segs &&
 			    sectors < max_sectors) {
 				/* split in the middle of bvec */
 				bv.bv_len = (max_sectors - sectors) << 9;
 				bvec_split_segs(q, &bv, &nsegs,
 						&seg_size,
 						&front_seg_size,
-						&sectors);
+						&sectors, max_segs);
 			}
 			goto split;
 		}
···
 			continue;
 		}
 new_segment:
-		if (nsegs == queue_max_segments(q))
+		if (nsegs == max_segs)
 			goto split;
 
 		bvprv = bv;
···
 			if (nsegs == 1 && seg_size > front_seg_size)
 				front_seg_size = seg_size;
 		} else if (bvec_split_segs(q, &bv, &nsegs, &seg_size,
-				    &front_seg_size, &sectors)) {
+				    &front_seg_size, &sectors, max_segs)) {
 			goto split;
 		}
 	}
···
 			bvprv = bv;
 			prev = 1;
 			bvec_split_segs(q, &bv, &nr_phys_segs, &seg_size,
-					&front_seg_size, NULL);
+					&front_seg_size, NULL, UINT_MAX);
 		}
 		bbio = bio;
 	}