shared/gatt-server: Fix notification & indication packet size
shared/gatt-server: Fix notification & indication packet size

For both Handle Value Notification & Handle Value Indication, the size
of the payload is limited to (ATT_MTU - 3) bytes, as indicated in Vol 3,
part F, section 3.4.7.1 & 3.4.7.2 of the Bluetooth 4.2 specification.

In the function, the first reserved byte is implicit and corresponds to
the opcode, while the 2 next bytes are explicit and contain the
attribute handle.

Ensure that the truncation of large payloads is done with the correct
value.

pstglia/external-bluetooth-bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,mapfau/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez