Use fine-grained mutex for stack.cleanupEndpoints.
Use fine-grained mutex for stack.cleanupEndpoints.

stack.cleanupEndpoints is protected by the stack.mu but that can cause
contention as the stack mutex is already acquired in a lot of hot paths during
new endpoint creation /cleanup etc. Moving this to a fine grained mutex should
reduce contention on the stack.mu.

PiperOrigin-RevId: 330026151

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