Bluetooth: Fix a buffer overflow in mgmt_mesh_add()

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: Fix a buffer overflow in mgmt_mesh_add()

Smatch Warning:

net/bluetooth/mgmt_util.c:375 mgmt_mesh_add() error: __memcpy()

'mesh_tx->param' too small (48 vs 50)

Analysis:

'mesh_tx->param' is array of size 48. This is the destination.

u8 param[sizeof(struct mgmt_cp_mesh_send) + 29]; // 19 + 29 = 48.

But in the caller 'mesh_send' we reject only when len > 50.

len > (MGMT_MESH_SEND_SIZE + 31) // 19 + 31 = 50.