gatt-server: Flush notify multiple buffer when full and fix overflow

gatt-server: Flush notify multiple buffer when full and fix overflow

This fixes the calculation of available buffer space in
bt_gatt_server_send_notification and sends pending notifications
immediately when there is no more room to add a notification.

Previously there was a buffer overflow caused by incorrect calculation
of available buffer space: data->offset can equal data->len
from a previous call to this function, leading
(data->len - data->offset) to underflow after data->offset += 2.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>