Bluetooth: Mesh: Fix reference count imbalance in bt_mesh_net_resend()

The commit 1c7b6688 tried to fix
resending segments to the GATT bearer, however it got the buffer
refernce counting wrong. The bt_mesh_net_resend() function doesn't
own a reference to the buffer, i.e. it's not responsible for unrefing
it. E.g. bt_mesh_adv_send() takes its own reference.

What was missing however was the handling of the callbacks. Use the
recently introduced send_cb_finalize() helper to make sure they're
called.

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