Automatic merge from submit-queue (batch tested with PRs 42942, 42935)

pkg/util/flock: Fix the flock so it actually locks.

With this PR, the second call to `Acquire()` will block unless the lock is released (process exits).
Also removed the memory mutex in the previous code since we don't need `Release()` here so no need to save and protect the local fd.

Fix #42929.