dtrace: handle NOP instructions in RISC-V

[christos]

Summary

dtrace_invop_start() was panicking if the provider returned
MATCH_C_NOP (DTRACE_INVOP_NOP).

[markj]

sys/cddl/dev/dtrace/riscv/dtrace_subr.c

275

It looks like fbt_invop() returns the original instruction, rather than using the DTRACE_INVOP_* constants. In particular, fbt->fbt_rval is not used anywhere.

We should thus return a nop instruction and match it the way other instructions are matched, rather than having multiple return types.