linux: Fix ptrace panic with ERESTART

Translate ERESTART into Linux "internal" errno ERESTARTSYS.
This fixes the erestartsys.gen.test from strace(1).

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32623

(cherry picked from commit 6547153e4618c3b57e5f76062de006a04ecbd64b)