linux: Adjust PTRACE_GET_SYSCALL_INFO buffer size semantics

The tests/ptrace_syscall_info test from strace(1) complained
about this.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32368

(cherry picked from commit 75a9d95b4d0a043ac0f3f8d1efc0982156337eac)