ctl: Use ctl_io_sbuf in ctl_process_done

This reduces a second copy of (mostly) the same code.

Reviewed by: ken, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D42210

(cherry picked from commit e846a3e0168f9188a6eccedd52707c63fd83cba2)