net: buf: Clear `user_data` on allocation

For two reasons:
- prevent exposing the previous user's info to the new user
- make NULL checks on user_data work

Since we don't really have a field that specifies what part of the
user_data array is valid, we have to rely on other checks.

Such a check, if user_data contains a callback, is comparing against
NULL before calling said callback.

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