Pin goroutine to OS thread when reading the primary key

[gerald-yang-tw]

The keyring links established by KEYCTL_LINK are per-thread, but Go may reschedule the goroutine onto a different OS thread between the link and the subsequent key read, making the read intermittently fail with permission denied:
final reseal failed: cannot obtain auth key from kernel: cannot read key from kernel: cannot determine size of key payload: permission denied

Wrap the link and read in runtime.LockOSThread()/UnlockOSThread() so both syscalls run on the same thread.

[gerald-yang-tw]

This issue can be reproduced on a azure CVM instance by running the following script:

#!/bin/bash

for i in {1..100}; do
if ! sudo apt reinstall nullboot; then
echo " ##### ITERATION #$i #####"
echo "reproduced the issue"
exit 1
fi
echo " ##### ITERATION #$i #####"
done