bcachefs: Fix a null ptr deref during journal replay

We were calling bch2_extent_can_insert() incorrectly; it should only be
called when the extents-to-keys pass is running because that's when we
could be splitting a compressed extent. Calling bch2_extent_can_insert()
without passing in a disk reservation was causing a null ptr deref.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update_leaf.c

@@ -309,7 +309,7 @@ btree_key_can_insert(struct btree_trans *trans,
 	if (unlikely(btree_node_old_extent_overwrite(b)))
 		return BTREE_INSERT_BTREE_NODE_FULL;
 
-	ret = !btree_node_is_extents(b)
+	ret = !(iter->flags & BTREE_ITER_IS_EXTENTS)
 		? BTREE_INSERT_OK
 		: bch2_extent_can_insert(trans, iter, insert);
 	if (ret)