Bluetooth: L2CAP: add TX timestamping

Support TX timestamping in L2CAP sockets.

Support MSG_ERRQUEUE recvmsg.

For other than SOCK_STREAM L2CAP sockets, if a packet from sendmsg() is
fragmented, only the first ACL fragment is timestamped.

For SOCK_STREAM L2CAP sockets, use the bytestream convention and
timestamp the last fragment and count bytes in tskey.

Timestamps are not generated in the Enhanced Retransmission mode, as
meaning of COMPLETION stamp is unclear if L2CAP layer retransmits.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>