Bluetooth: Add l2cap_state_change_and_error()

l2cap_state_change_and_error() introduces the ability to update a
l2cap_user with changes in channel's state and error code with just one
call. The main reason for this is to avoid race conditions between and
setting the state and then the error. Otherwise we would need to release
the lock between both operations.

This is another step of an ongoing work to make l2cap_core.c totally
independent from l2cap's struct sock.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>