Drivers: vmbus: Check for channel allocation before looking up relids

In the Linux kernel, the following vulnerability has been resolved:

Drivers: vmbus: Check for channel allocation before looking up relids

relid2channel() assumes vmbus channel array to be allocated when called. However, in cases such as kdump/kexec, not all relids will be reset by the host. When the second kernel boots and if the guest receives a vmbus interrupt during vmbus driver initialization before vmbus_connect() is called, before it finishes, or if it fails, the vmbus interrupt service routine is called which in turn calls relid2channel() and can cause a null pointer dereference.

Print a warning and error out in relid2channel() for a channel id that's invalid in the second kernel.