btrfs: scrub: simplify cleanup of wr_ctx in scrub_free_ctx

We don't need to take the mutex and zero out wr_cur_bio, as this is
called after the scrub finished.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -639,11 +639,6 @@ static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
 	if (!sctx)
 		return;
 
-	mutex_lock(&sctx->wr_ctx.wr_lock);
-	kfree(sctx->wr_ctx.wr_curr_bio);
-	sctx->wr_ctx.wr_curr_bio = NULL;
-	mutex_unlock(&sctx->wr_ctx.wr_lock);
-
 	/* this can happen when scrub is cancelled */
 	if (sctx->curr != -1) {
 		struct scrub_bio *sbio = sctx->bios[sctx->curr];
@@ -663,6 +658,7 @@ static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
 		kfree(sbio);
 	}
 
+	kfree(sctx->wr_ctx.wr_curr_bio);
 	scrub_free_csums(sctx);
 	kfree(sctx);
 }