powerpc/pseries/iommu: Don't unset window if it was never set

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

powerpc/pseries/iommu: Don't unset window if it was never set

On pSeries, when user attempts to use the same vfio container used by different iommu group, the spapr_tce_set_window() returns -EPERM and the subsequent cleanup leads to the below crash.

Kernel attempted to read user page (308) - exploit attempt? BUG: Kernel NULL pointer dereference on read at 0x00000308 Faulting instruction address: 0xc0000000001ce358 Oops: Kernel access of bad area, sig: 11 [#1] NIP: c0000000001ce358 LR: c0000000001ce05c CTR: c00000000005add0

NIP [c0000000001ce358] spapr_tce_unset_window+0x3b8/0x510 LR [c0000000001ce05c] spapr_tce_unset_window+0xbc/0x510 Call Trace: spapr_tce_unset_window+0xbc/0x510 (unreliable) tce_iommu_attach_group+0x24c/0x340 [vfio_iommu_spapr_tce] vfio_container_attach_group+0xec/0x240 [vfio] vfio_group_fops_unl_ioctl+0x548/0xb00 [vfio] sys_ioctl+0x754/0x1580 system_call_exception+0x13c/0x330 system_call_vectored_common+0x15c/0x2ec

--- interrupt: 3000

Fix this by having null check for the tbl passed to the spapr_tce_unset_window().