FileHandle is not thread-safe on native due to no-op Lock implementation.

[MohammedKHC]

FileHandle uses Lock.withLock() to protect closed and openStreamCount, but on native platforms the Lock implementation does nothing:

okio/okio/src/nonJvmMain/kotlin/okio/NonJvmPlatform.kt

Lines 42 to 48 in 79aa267

	actual inline fun <T> Lock.withLock(action: () -> T): T {
	contract {
	callsInPlace(action, InvocationKind.EXACTLY_ONCE)
	}
	return action()
	}

This means FileHandle has race conditions when used from multiple threads.

• openStreamCount can be corrupted
• Resource leaks possible when close() races with source()/sink()
• Methods check !closed but state can change between check and use

Can we use AtomicBoolean and AtomicInt instead of Lock?
Or implement it on native using pthread_mutex?