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.

Reviewed by: melifaro (earlier version), markj, gallatin
Sponsored by: Netflix (partially, originally)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23740