btrfs: scrub: fix target device intialization while setting up scrub context

The commit "btrfs: scrub: inline helper scrub_setup_wr_ctx" inlined a
helper but wrongly sets up the target device. Incidentally there's a
local variable with the same name as a parameter in the previous
function, so this got caught during runtime as crash in test btrfs/027.
Reported-by: NChris Mason <clm@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -714,9 +714,9 @@ struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
 	mutex_init(&sctx->wr_lock);
 	sctx->wr_curr_bio = NULL;
 	if (is_dev_replace) {
-		WARN_ON(!dev->bdev);
+		WARN_ON(!fs_info->dev_replace.tgtdev);
 		sctx->pages_per_wr_bio = SCRUB_PAGES_PER_WR_BIO;
-		sctx->wr_tgtdev = dev;
+		sctx->wr_tgtdev = fs_info->dev_replace.tgtdev;
 		atomic_set(&sctx->flush_all_writes, 0);
 	}