nvme-pci: fix use-after-free in nvme_free_host_mem()

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

nvme-pci: fix use-after-free in nvme_free_host_mem()

nvme_free_host_mem() frees dev->hmb_sgt via dma_free_noncontiguous()
but never clears the pointer afterward. This leads to a use-after-free
if nvme_free_host_mem() is called twice in the same error path.

This can happen during nvme_probe() when nvme_setup_host_mem() succeeds
in allocating the HMB (setting dev->hmb_sgt) but nvme_set_host_mem()
fails with an I/O error:

nvme_setup_host_mem()
nvme_alloc_host_mem_single() -> sets dev->hmb_sgt
nvme_set_host_mem() -> fails with -EIO
nvme_free_host_mem() -> frees hmb_sgt, but does NOT NULL it
return error

nvme_probe() error path:
nvme_free_host_mem() -> dev->hmb_sgt is stale, use-after-free

The second call dereferences the freed sgt, causing a NULL pointer
dereference in iommu_dma_free_noncontiguous() when it accesses
sgt->sgl->dma_address (the backing memory has been freed and zeroed).

This is reproducible on Thunderbolt-attached NVMe devices (e.g., OWC
Envoy Express behind a Dell WD22TB4 dock) where the device intermittently
returns I/O errors during HMB setup due to PCIe link instability.

BUG: kernel NULL pointer dereference, address: 0000000000000010
RIP: 0010:iommu_dma_free_noncontiguous+0x22/0x80
Call Trace:
<TASK>
dma_free_noncontiguous+0x3b/0x130
nvme_free_host_mem+0x30/0xf0 [nvme]
nvme_probe.cold+0xcc/0x275 [nvme]
local_pci_probe+0x43/0xa0
pci_device_probe+0xeea/0x290
really_probe+0xf9/0x3b0
__driver_probe_device+0x8b/0x170
driver_probe_device+0x24/0xd0
__driver_attach_async_helper+0x6b/0x110
async_run_entry_fn+0x37/0x170
process_one_work+0x1ac/0x3d0
worker_thread+0x1b8/0x360
kthread+0xf7/0x130
ret_from_fork+0x2d8/0x3a0
ret_from_fork_asm+0x1a/0x30
</TASK>

Fix this by setting dev->hmb_sgt to NULL after freeing it, so the
second call takes the multi-descriptor path which safely handles the
already-cleaned-up state.