vfio: Fix NULL pointer dereference caused by uninitialized group->iommufd

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

vfio: Fix NULL pointer dereference caused by uninitialized group->iommufd

group->iommufd is not initialized for the iommufd_ctx_put()

[20018.331541] BUG: kernel NULL pointer dereference, address: 0000000000000000

[20018.377508] RIP: 0010:iommufd_ctx_put+0x5/0x10 [iommufd]

...

[20018.476483] Call Trace:

[20018.479214] <TASK>

[20018.481555] vfio_group_fops_unl_ioctl+0x506/0x690 [vfio]

[20018.487586] __x64_sys_ioctl+0x6a/0xb0

[20018.491773] ? trace_hardirqs_on+0xc5/0xe0

[20018.496347] do_syscall_64+0x67/0x90

[20018.500340] entry_SYSCALL_64_after_hwframe+0x4b/0xb5