darwin: add missing mutex lock/unlock for claimed_interfaces

[seanm]

Existing code comments say that
libusb_device_handle.claimed_interfaces is protected by libusb_device_handle.lock. The function ep_to_pipeRef() accesses claimed_interfaces but does not hold the lock.

I examined all 9 callers:

• darwin_alloc_streams
• darwin_clear_halt
• darwin_free_streams
• darwin_abort_transfers
• submit_bulk_transfer
• submit_control_transfer
• submit_iso_transfer
• submit_stream_transfer
• darwin_async_io_callback

and none seem to hold the lock already.

Found with -Wthread-safety.

[seanm]

@hjelmn does this look right to you?

[seanm]

@osy could you take a look at this?

[osy]

This is a fix for a read race on claimed_interface (not write race) right? It looks fine to me but just a heads up, when I looked at the Darwin backend, I found a lot of read races and just reasoned that all of them would result in eventual errors that is similar to if the device was just hot unplugged.

[seanm]

@osy Yes reading. This change is as a result of a -Wthread-safety compiler warning. According to existing comments:

	/* lock protects claimed_interfaces */
	usbi_mutex_t lock;
	unsigned long claimed_interfaces;

claimed_interfaces should be protected by lock.