Drivers: hv: vmbus: Fix handling of messages with transaction ID of zero

Drivers: hv: vmbus: Fix handling of messages with transaction ID of zero

vmbus_request_addr() returns 0 (zero) if the transaction ID passed
to as argument is 0.  This is unfortunate for two reasons: first,
netvsc_send_completion() does not check for a NULL cmd_rqst (before
dereferencing the corresponding NVSP message); second, 0 is a *valid*
value of cmd_rqst in netvsc_send_tx_complete(), cf. the call of
vmbus_sendpacket() in netvsc_send_pkt().

vmbus_request_addr() has included the code in question since its
introduction with commit e8b7db38449ac ("Drivers: hv: vmbus: Add
vmbus_requestor data structure for VMBus hardening"); such code was
motivated by the early use of vmbus_requestor by hv_storvsc.  Since
hv_storvsc moved to a tag-based mechanism to generate and retrieve
transaction IDs with commit bf5fd8cae3c8f ("scsi: storvsc: Use
blk_mq_unique_tag() to generate requestIDs"), vmbus_request_addr()
can be modified to return VMBUS_RQST_ERROR if the ID is 0.  This
change solves the issues in hv_netvsc (and makes the handling of
messages with transaction ID of 0 consistent with the semantics
"the ID is not contained in the requestor/invalid ID").

vmbus_next_request_id(), vmbus_request_addr() should still reserve
the ID of 0 for Hyper-V, because Hyper-V will "ignore" (not respond
to) VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED packets/requests with
transaction ID of 0 from the guest.

Fixes: bf5fd8cae3c8f ("scsi: storvsc: Use blk_mq_unique_tag() to generate requestIDs")
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220419122325.10078-2-parri.andrea@gmail.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>