Use the known valid segment when accessing memory in #UD handler.
Make sure that %eflags.D flag is cleared for hook.
Improve comments.

When #UD dtrace code checks for a registered hook before checking that
the exception was raised from kernel mode, we might run with the user
%ds, trapping on access. Exception entry from userspace automatically
load valid %ss, which we can use there instead.

Noted and reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 3 days