Use CSUM_BITS instead of incorrect copy.

Sponsored by:	Nginx, Inc.
Clean up unused CSUM_FRAGMENT.

Sponsored by:	Nginx, Inc.
Retire thread_unthread(), it has only one caller.  Update comment in
the block of code before the previous call to thread_unthread().

Discussed with:	alc
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Style.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week