vfio/cdx: Consolidate MSI configured state onto cdx_irqs

vfio/cdx: Consolidate MSI configured state onto cdx_irqs

struct vfio_cdx_device carries three fields that track whether MSI has
been configured: vdev->cdx_irqs (the allocated vector array), vdev->
msi_count (the array length), and vdev->config_msi (a boolean flag).
The three are set together when vfio_cdx_msi_enable() succeeds and
cleared together by vfio_cdx_msi_disable().  However, the error paths
in vfio_cdx_msi_enable() free the cdx_irqs allocation on failure
without resetting the pointer, leaving it stale and skewed from the
other two fields until the next enable call overwrites it.

Clear vdev->cdx_irqs to NULL alongside the kfree() in both error paths
so the pointer consistently reflects the configured state.  With that
invariant restored and access to the MSI state serialized by
cdx_irqs_lock, vdev->config_msi is fully redundant with
(vdev->cdx_irqs != NULL).  Drop the config_msi field and switch all
readers to test cdx_irqs directly.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Acked-by: Nikhil Agarwal <nikhil.agarwal@amd.com>
Link: https://lore.kernel.org/r/20260417202800.88287-4-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>