Drivers: hv: vmbus: Export hv_vmbus_exists() and use it in pci-hyperv

With commit f84b21da3624 ("PCI: hv: Don't load the driver for baremetal root partition"),
the bare metal Linux root partition won't use the pci-hyperv driver, but
when a Linux VM runs on the Linux root partition, pci-hyperv's module_init
function init_hv_pci_drv() can still run, e.g. in the case of
CONFIG_PCI_HYPERV=y, even if the VMBus driver is not used in such a VM
(i.e. the hv_vmbus driver's init function returns -ENODEV due to
vmbus_root_device being NULL).

In such a Linux VM, init_hv_pci_drv() runs with a side effect: the 3
hvpci_block_ops callbacks are set to functions that depend on hv_vmbus.

Later, when the MLX driver in such a VM invokes the callbacks, e.g. in
drivers/net/ethernet/mellanox/mlx5/core/lib/hv.c:
mlx5_hv_register_invalidate(), hvpci_block_ops.reg_blk_invalidate() is
hv_register_block_invalidate() rather than a NULL function pointer, and
hv_register_block_invalidate() assumes that it can find a struct
hv_pcibus_device from pdev->bus->sysdata, which is false in such a VM.

Consequently, hv_register_block_invalidate() -> get_pcichild_wslot() ->
spin_lock_irqsave() may hang since it can be accessing an invalid
spinlock pointer.

Fix the issue by exporting hv_vmbus_exists() and using it in pci-hyperv:

hv_root_partition() is true and hv_nested is false ==>
hv_vmbus_exists() is false.

hv_root_partition() is true and hv_nested is true ==>
hv_vmbus_exists() is true.

hv_root_partition() is false ==> hv_vmbus_exists() is true.

While at it, rename vmbus_exists() to hv_vmbus_exists() to follow the
convention that all public functions have the hv_ prefix; also change
the return value's type from int to bool to make the code more readable;
also move the two pr_info() calls.

Reported-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>