On systems or VMs where multiple devices share a single DevID
(because they sit behind a PCI bridge, or because the HW is
broken in funky ways), we reuse the save its_device structure
in order to reflect this.

It turns out that there is a distinct lack of locking when looking
up the its_device, and two device being probed concurrently can result
in double allocations. That's obviously not nice.

A solution for this is to have a per-ITS mutex that serializes device
allocation.

A similar issue exists on the freeing side, which can run concurrently
with the allocation. On top of now taking the appropriate lock, we
also make sure that a shared device is never freed, as we have no way
to currently track the life cycle of such object.

Reported-by: Zheng Xiang <zhengxiang9@huawei.com>
Tested-by: Zheng Xiang <zhengxiang9@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>