Bluetooth: Mesh: Fix Assert in bt_mesh_adv_unref when messages to a proxy

Fixes:https://github.com/zephyrproject-rtos/zephyr/issues/83904



This solution fix is to define a separate variable for the each proxy FIFO.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>
(cherry picked from commit 63710804)