Claude review: vfio/pci: Add a helper to look up PFNs for DMABUFs

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Adds `vfio_pci_dma_buf_find_pfn()` for translating fault addresses to PFNs via the DMABUF's `phys_vec[]` ranges.

**The comment block is quite long (lines 438-470)**. A 30+ line comment preceding the function duplicates what the code already communicates. Consider trimming to just the non-obvious parts (the `vma_pgoff_adjust` semantics and the hugepage/range-boundary limitation).

**The `vma_pgoff_adjust` field and its WARN**: 
```c
+	if (vpdmabuf->vma_pgoff_adjust != 0 &&
+	    vpdmabuf->vma_pgoff_adjust != (vma->vm_pgoff &
+					   (VFIO_PCI_OFFSET_MASK >> PAGE_SHIFT))) {
+		WARN(1, "Unexpected vma_pgoff_adjust 0x%lx (vm_pgoff 0x%lx)\n",
```
This WARN fires if `vma_pgoff_adjust` is nonzero and doesn't match the VMA's pgoff. The check is a debugging assertion that should arguably be a `WARN_ON_ONCE` rather than `WARN(1, ...)` to avoid spamming dmesg.

**Struct movement**: Moving `struct vfio_pci_dma_buf` to the header and adding `vma_pgoff_adjust` is clean. Good that `vma_pgoff_adjust` is `unsigned long` to match `vm_pgoff`.

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