Fix buffer leak in L2CAP and SMP when attempting to send over a
a connection that has been disconnected.
Both L2CAP and SMP now have ownership of the buffer in case of failure
to send it and need to unref it in case of send failure.
Change the return type of bt_l2cap_send since the user of this function
has to handle releasing of the buffer when this fails.
Regression from: a77f809faa47ff9922692d2b4eab33062b8a2b4d
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>