Fix lockfiles never detecting changes

If we ever check the last-writer of a lock file and get back a zero
value, don't only check if subsequent last-writer values match its first
zero bytes, which is always true, as a test to see if the last-writer
value has remained unchanged.

Signed-off-by: Nalin Dahyabhai <[email protected]>

pkg/lockfile/lockfile_unix.go

@@ -246,7 +246,7 @@ func (l *lockfile) Modified() (bool, error) {
 		panic("attempted to check last-writer in lockfile without locking it first")
 	}
 	defer l.stateMutex.Unlock()
-	currentLW := make([]byte, len(l.lw))
+	currentLW := make([]byte, lastWriterIDSize)
 	n, err := unix.Pread(int(l.fd), currentLW, 0)
 	if err != nil {
 		return true, err