linux: Add additional ptracestop only if the debugger is Linux

[trasz]

Summary

In 6e66030c4c0, additional ptracestop was added in order
to implement PTRACE_EVENT_EXEC. Make it only apply to cases where the debugger is a Linux processes;
native FreeBSD debuggers can trace Linux processes too,
but they don't expect that additonal ptracestop.

Fixes: 6e66030c4c0
Reported By: kib
Sponsord By: EPSRC

[kib]

sys/kern/subr_syscall.c

274

I am not sure of the check for ABI(p) is needed. Or rather, I think that it is not needed, and I do not see arguments that would support keeping this check.

275

Note that p->p_pptr cannot change while p is locked. ABI of p_pptr can, but I think it does not matter if we loose the race that way.

In other words, I think that locking debugger is not needed there (but a comment why we skip the locking is needed).

[trasz]

Fix issues pointed out by kib@.

[kib]

sys/kern/subr_syscall.c

267

I would formulate it differently. If debugger changes its ABI while we are returning from syscall, factually execs, then there is a race which is not handled anyway. We might win this race, or debugger might, outcome is not deterministic, and our choice to add pptr lock does not solve the race.

[trasz]

Tweak the comment.