bcachefs: Fix an out-of-bounds shift

roundup_pow_of_two() is undefined for 0 - oops.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2879,7 +2879,7 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c, unsigned fn_
 	bch2_trans_alloc_paths(trans, c);
 
 	s = btree_trans_stats(trans);
-	if (s) {
+	if (s && s->max_mem) {
 		unsigned expected_mem_bytes = roundup_pow_of_two(s->max_mem);
 
 		trans->mem = kmalloc(expected_mem_bytes, GFP_KERNEL);
@@ -2890,9 +2890,9 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c, unsigned fn_
 		} else {
 			trans->mem_bytes = expected_mem_bytes;
 		}
-
-		trans->nr_max_paths = s->nr_max_paths;
 	}
+	if (s)
+		trans->nr_max_paths = s->nr_max_paths;
 
 	trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);