Require sync.Mutex to lock and unlock from the same goroutine

We would like to track locks ordering to detect ordering violations. Detecting
violations is much simpler if mutexes must be unlocked by the same goroutine
that locked them.

Thus, as a first step to tracking lock ordering, add this lock/unlock
requirement to gVisor's sync.Mutex. This is more strict than the Go standard
library's sync.Mutex, but initial testing indicates only a single lock that is
used across goroutines. The new sync.CrossGoroutineMutex relaxes the
requirement (but will not provide lock order checking).

Due to the additional overhead, enforcement is only enabled with the
"checklocks" build tag. Build with this tag using:

bazel build --define=gotags=checklocks ...

From my spot-checking, this has no changed inlining properties when disabled.

Updates #4804

PiperOrigin-RevId: 343370200

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