wifi: cfg80211: allow cfg80211_defragment_element() without output

wifi: cfg80211: allow cfg80211_defragment_element() without output

If we just want to determine the length of the fragmented
data, we basically need the same logic, and really we want
it to be _literally_ the same logic, so it cannot be out
of sync in any way.

Allow calling cfg80211_defragment_element() without an output
buffer, where it then just returns the required output size.

Also add this to the tests, just to exercise it, using the
pre-calculated length to really do the defragmentation, which
checks that this is sufficient.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://msgid.link/20240228095718.6d6565b9e3f2.Ib441903f4b8644ba04b1c766f90580ee6f54fc66@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>