Bluetooth: L2CAP: Downgrade user_data API error to a warning

When user_data is not zeroed-out, the API returns an error. Downgrade
the API error to a warning log instead.

Introducing this check (#76489) broke a few PTS tests, as user_data is
not initialized by `net_buf_alloc()`. Doing so is in discussion:

https://github.com/zephyrproject-rtos/zephyr/issues/77088

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
(cherry picked from commit 6fa6c4c256e7763e1cb97974f25e70413cc95c99)
6 files changed