Bluetooth: Mesh: Thread stack can reduce by use malloc&free function

[LingaoM]

zephyr/subsys/bluetooth/mesh/transport.c

Lines 610 to 615 in f79fbac

	static int sdu_recv(struct bt_mesh_net_rx *rx, u32_t seq, u8_t hdr,
	u8_t aszmic, struct net_buf_simple *buf)
	{
	NET_BUF_SIMPLE_DEFINE(sdu, CONFIG_BT_MESH_RX_SDU_MAX - 4);
	u8_t *ad;
	u16_t i;

Just as above code, when sdu_recv be called, stack will occupy at least CONFIG_BT_MESH_RX_SDU_MAX octers, this problem occurs in many places. I think we can replace a large number of stack uses with heap.

[jhedberg]

Zephyr doesn't have a heap by default, and neither the Bluetooth stack nor the mesh code requires it. Creating a heap would therefore initially likely consume more memory than the current code. Additionally the current mempool-based allocator isn't super efficient. I remember seeing some work toward a better heap allocator, so we could potentially reconsider this in the future.

Another drawback of moving to heap is that we have to start handling allocation errors everywhere, whereas now we're always guaranteed to have the memory available.