Bluetooth: Mesh: allocate mesh max required buffer number

Since sending of public key was moved into system
work (https://github.com/zephyrproject-rtos/zephyr/pull/62331

)
it uses the same context as a Host Tx buffer
allocator for gatt sending. Host cannot wait for
free buffer anymore. Mesh requires 4 buffers
to send max size frame(public key) during
provisioning.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>