bcache: Use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
drivers/md/bcache/btree.c:1800:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci
Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -1804,10 +1804,7 @@ static int bch_gc_thread(void *arg)
 int bch_gc_thread_start(struct cache_set *c)
 {
 	c->gc_thread = kthread_run(bch_gc_thread, c, "bcache_gc");
-	if (IS_ERR(c->gc_thread))
-		return PTR_ERR(c->gc_thread);
-
-	return 0;
+	return PTR_ERR_OR_ZERO(c->gc_thread);
 }
 
 /* Initial partial gc */