dtrace/amd64: Fix probe argument fetching

dtrace_getarg() previously walked the call stack looking for a frame
matching the dtrace_invop_callsite symbol, in order to look for a
trapframe corresponding to an invop (i.e., FBT or kinst) probe. Commit
3ba8e9dc4a0e broke this in some cases by breaking the expected alignment
of the dtrace_invop_callsite symbol.

Rather than groveling around the stack to find invop probe arguments,
simply use the trapframe reference saved by dtrace_invop(). This is
simpler and less fragile.

Reported by: avg
Reviewed by: avg
MFC after: 2 weeks
Fixes: 3ba8e9dc4a0e ("dtrace/amd64: Implement emulation of call instructions")
Differential Revision: https://reviews.freebsd.org/D46672

(cherry picked from commit 9aabab09c4e163fa8187b265d6836d822516b1c9)