linux: make ptrace(2) return EIO when trying to peek invalid address

Previously we've returned the error from native ptrace(2), ENOMEM.
This confused Linux strace(2).

Reviewed By: emaste
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29925

(cherry picked from commit 77651151f3caec7d2440404f8c0f1819757d1d5d)