MFC r358167,358311: ip6_output: improve extension header handling
Move IPv6 source address checks from after extension header heandling to the top of the function. If we do not pass these checks there is no reason to do a lot of work upfront. Fold extension header preparations and length calculations together into a single branch and macro rather than doing them sequentially. Likewise move extension header concatination into a single branch block only doing it if we recorded any extension header length length. Sponsored by: Netflix (partially, originally)