geom: Report copyout() errors in g_ctl_ioctl_ctl()

Despite the name, req->serror is used in some cases to copy non-error
messages to userspace. So, report errors when copying out so long as
they don't clobber an earlier error.

Reviewed by: mav, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43146

(cherry picked from commit bbf221e3e822d15dafb37bd8e200a9864f2636b4)