dtrace: fix fbt return probes on RISC-V

Return values are passed in a0, so read it from there. We also pass a1 through
to userspace, as the ABI allows small structs to be returned in registers
a0/a1. While here read the register values directly from the trapframe rather
than rtval, and remove the now unneeded argument from dtrace_invop().

Set fbtp_roffset so that we get the correct return location in arg0.

Reviewed by: markj
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D26389