Redo r360682, now with only a minimal fix for misleading indentation:

MFC r333465 (partial, by lwhsu):

Fix build for platforms using GCC:

[omitted] - Remove unused or dead store variable
[omitted] - Remove unused function ctl_copyin_alloc

• Add missing curly brackets, this seems a regression in r287720

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D15383