kinst/arm64: Fix return values from kinst_invop()

[markj]

Summary

After commit 853cd8723494 it became invalid for kinst_invop() to return
0: dtrace_invop_start() would convert this to a sentinel value
indicating that it did not consume the breakpoint, and so we'd just
call kdb_trap() to handle it.

Change kinst_invop() to return NOP_INSTR after handling a matching
breakpoint. NOP_INSTR is handled by advancing the ELR, so we have to
compensate by subtracting INSTR_SIZE before returning.

Fixes: 853cd8723494 ("arm64: Clean up usage of the dtrace invop handler")

[christos]

sys/cddl/dev/kinst/aarch64/kinst_isa.c

233–234

I think it'd be good to include the explanation from the commit message here as well. After a while it might not be obvious what we meant here.

[markj]

sys/cddl/dev/kinst/aarch64/kinst_isa.c

233–234

It seems pretty clear if one actually looks at dtrace_invop_start()...?

[christos]

sys/cddl/dev/kinst/aarch64/kinst_isa.c

233–234

Yes, I just thought it might be handy to have a short explanation here as well.

[markj]

use more words