Bluetooth: Mesh: Fix NULL pointer access with GATT Proxy

The conversion to the new net_buf_simple API was done incorrectly
here. The buffer initialization should use net_buf_simple_init()
instead of net_buf_simple_reset(), so that buf->__buf gets properly
initialized (and not left pointing at NULL).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>