Fix lock order violation.
Fix lock order violation.

overlayFileOperations.Readdir was holding overlay.copyMu while calling
DirentReaddir, which then attempts to take take the corresponding Dirent.mu,
causing a lock order violation. (See lock order documentation in
fs/copy_up.go.)

We only actually need to hold copyMu during readdirEntries(), so holding the
lock is moved in there, thus avoiding the lock order violation.

A new lock was added to protect overlayFileOperations.dirCache. We were
inadvertently relying on copyMu to protect this.  There is no reason it should
not have its own lock.

PiperOrigin-RevId: 228542473
Change-Id: I03c3a368c8cbc0b5a79d50cc486fc94adaddc1c2

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor