exec: Remove parameter 'segflg' from exec_copyin_args()

In kern "copyin" means copy data from user address space to kernel
address space. But in the function exec_copyin_args() there is a
parameter 'segflg' that is used to specify the address space of the
parameter 'fname'. In the source code there are two places where
'segflg' are not UIO_USERSPACE. In both cases the 'fname' argument are
NULL so the argument 'segflg' are not important there. So it is safe to
remove the parameter 'segflg' from the function exec_copyin_args().

Reviewed by: markj, jhb
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1590

(cherry picked from commit 15581af7c2d30360313dcad74dc5dc83c02f9df0)