ip6_output: improve extension header handling

[bz]

Summary

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)
MFC after: 1 week

[melifaro]

LGTM.
It looks like the next step could potentially be isolation the whole if (opt){} and all its consequences into a separate function, filling in exthdr?

[markj]

sys/netinet6/ip6_output.c

158–159

You should remove the "and," or move it.

170

There should be parens around mp.

493

Why is this removed? After the ip6_splithdr() call, ip6 is left pointing at a stale copy of the header, no?

[bz]

Correct changes based on the comments from @markj and make hdrsplit
a bool as it is used as such.

[bz]

sys/netinet6/ip6_output.c

493

Good catch. Was extracted from another patch where the code has changed more. I put it into each branch after a successful ip6_splithdr() call. The ip6_insert_jumboopt() further down does not seem to need it to my reading.