bcache: put bch_bio_map() back to correct location in journal_write_unlocked()

Commit a7c50c9 ("block: pass a block_device and opf to bio_reset")
moves bch_bio_map() inside journal_write_unlocked() next to the location
where the modified bio_reset() was called.

This change is wrong because calling bch_bio_map() immediately after
bio_reset(), a BUG_ON(!bio->bi_iter.bi_size) inside bch_bio_map() will
be triggered and panic the kernel.

This patch puts bch_bio_map() back to its original correct location in
journal_write_unlocked() and avoid the BUG_ON().

Fixes: a7c50c9 ("block: pass a block_device and opf to bio_reset")
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220419160425.4148-2-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Coly Li

drivers/md/bcache/journal.c

@@ -771,12 +771,12 @@ static void journal_write_unlocked(struct closure *cl)
 
 		bio_reset(bio, ca->bdev, REQ_OP_WRITE | 
 			  REQ_SYNC | REQ_META | REQ_PREFLUSH | REQ_FUA);
-		bch_bio_map(bio, w->data);
 		bio->bi_iter.bi_sector	= PTR_OFFSET(k, i);
 		bio->bi_iter.bi_size = sectors << 9;
 
 		bio->bi_end_io	= journal_write_endio;
 		bio->bi_private = w;
+		bch_bio_map(bio, w->data);
 
 		trace_bcache_journal_write(bio, w->data->keys);
 		bio_list_add(&list, bio);