Print user_data and callback pointers.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

Werror fails the build on that function.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

Storing stuff in user_data? That's a paddlin'

We have been debugging issue after issue because ownership of this
"user" data is not clearly defined. Now it is. L2CAP owns the user_data
field entirely, as soon as `send()` is called.

Also add a warning and retval using CHECKIF.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

Modify the test so it checks that user_data is used and then cleared by
the stack.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

It's already done.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>

It was not being set, and thus if the user_data contained garbage from
before, then conn.c would attempt to call that garbage.

Static channels don't have this issue, as every "SDU" fits into one PDU.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Huajiang Zheng <nxf88597@lsv051208.swis.nl-cdc01.nxp.com>