frag6: leave a note about upper layer header checks TBD

Per sepcification the upper layer header needs to be within the first
fragment. The check was not done so far and there is an open review for
related work, so just leave a note as to where to put it.
Move the extraction of frag offset up to this as it is needed to determine
whether this is a first fragment or not.

MFC after: 3 weeks
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