bcachefs: fix for unmount hang

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -1257,6 +1257,7 @@ void bch2_dev_allocator_add(struct bch_fs *c, struct bch_dev *ca)
 
 void bch2_dev_allocator_quiesce(struct bch_fs *c, struct bch_dev *ca)
 {
+	if (ca->alloc_thread)
 		closure_wait_event(&c->freelist_wait, ca->allocator_blocked_full);
 }