bcachefs: Split out btree_error_wq

We can't use btree_update_wq becuase btree updates may be waiting on
btree writes to complete.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -667,6 +667,7 @@ struct bch_fs {
 	struct btree_key_cache	btree_key_cache;
 
 	struct workqueue_struct	*btree_update_wq;
+	struct workqueue_struct	*btree_error_wq;
 	/* copygc needs its own workqueue for index updates.. */
 	struct workqueue_struct	*copygc_wq;
 

fs/bcachefs/btree_io.c

@@ -1618,7 +1618,7 @@ static void btree_node_write_work(struct work_struct *work)
 		bio_list_add(&c->btree_write_error_list, &wbio->wbio.bio);
 		spin_unlock_irqrestore(&c->btree_write_error_lock, flags);
 
-		queue_work(c->btree_update_wq, &c->btree_write_error_work);
+		queue_work(c->btree_error_wq, &c->btree_write_error_work);
 		return;
 	}
 

fs/bcachefs/super.c

@@ -514,6 +514,8 @@ static void __bch2_fs_free(struct bch_fs *c)
 		destroy_workqueue(c->io_complete_wq );
 	if (c->copygc_wq)
 		destroy_workqueue(c->copygc_wq);
+	if (c->btree_error_wq)
+		destroy_workqueue(c->btree_error_wq);
 	if (c->btree_update_wq)
 		destroy_workqueue(c->btree_update_wq);
 
@@ -766,6 +768,8 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 
 	if (!(c->btree_update_wq = alloc_workqueue("bcachefs",
 				WQ_FREEZABLE|WQ_MEM_RECLAIM, 1)) ||
+	    !(c->btree_error_wq = alloc_workqueue("bcachefs_error",
+				WQ_FREEZABLE|WQ_MEM_RECLAIM, 1)) ||
 	    !(c->copygc_wq = alloc_workqueue("bcachefs_copygc",
 				WQ_FREEZABLE|WQ_MEM_RECLAIM|WQ_CPU_INTENSIVE, 1)) ||
 	    !(c->io_complete_wq = alloc_workqueue("bcachefs_io",