Bluetooth: LE L2CAP: Disconnect if received packet's SDU exceeds IMTU

Core 6.0, Vol 3, Part A, 3.4.3:
"If the SDU length field value exceeds the receiver's MTU, the receiver
shall disconnect the channel..."

This fixes L2CAP/LE/CFC/BV-26-C (running together with 'l2test -r -P
0x0027 -V le_public -I 100').

Fixes: aac23bf63659 ("Bluetooth: Implement LE L2CAP reassembly")
Signed-off-by: Christian Eggers <ceggers@arri.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>