[amd64] dtrace_invop handler is to be called only for kernel exceptions

DTrace-related exceptions in userland code are handled elsewhere.
One practical problem was a crash in dtrace_invop_start() when saved
%rsp pointed to a virtual address that was not backed.

i386 code already ignored userland exceptions.

Reviewed by: markj, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D5906