vfio/pci: fix memory leak during D3hot to D0 transition

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

vfio/pci: fix memory leak during D3hot to D0 transition

If 'vfio_pci_core_device::needs_pm_restore' is set (PCI device does

not have No_Soft_Reset bit set in its PMCSR config register), then

the current PCI state will be saved locally in

'vfio_pci_core_device::pm_save' during D0->D3hot transition and same

will be restored back during D3hot->D0 transition.

For saving the PCI state locally, pci_store_saved_state() is being

used and the pci_load_and_free_saved_state() will free the allocated

memory.

But for reset related IOCTLs, vfio driver calls PCI reset-related

API's which will internally change the PCI power state back to D0. So,

when the guest resumes, then it will get the current state as D0 and it

will skip the call to vfio_pci_set_power_state() for changing the

power state to D0 explicitly. In this case, the memory pointed by

'pm_save' will never be freed. In a malicious sequence, the state changing

to D3hot followed by VFIO_DEVICE_RESET/VFIO_DEVICE_PCI_HOT_RESET can be

run in a loop and it can cause an OOM situation.

This patch frees the earlier allocated memory first before overwriting

'pm_save' to prevent the mentioned memory leak.