Don't access userspace directly from the kernel in nxge(4).

Update to what the previous code seemed to be doing via the correct
interfaces. Further issues exist in xge_ioctl_registers(), but this is
debugging code in a driver that has few users and they don't appear to
be crashes or leaks.

Reviewed by: jhb (prior version)
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14848