Claude review: vfio/pci: Convert BAR mmap() to use a DMABUF

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Patch Review

The main conversion patch. Removes `vma_to_pfn()`, updates the fault handler, and makes `VFIO_PCI_CORE` depend on `DMA_SHARED_BUFFER` and `PCI_P2PDMA_CORE`.

**Kconfig change**:
```c
+	select PCI_P2PDMA_CORE
+	select DMA_SHARED_BUFFER
```
Making these unconditional `select`s for `VFIO_PCI_CORE` is a significant dependency change. `DMA_SHARED_BUFFER` is generally available, but this should be called out in the commit message more prominently since it affects all VFIO PCI users.

**Conditional `.attach` in dma_buf_ops**:
```c
 static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
+#ifdef CONFIG_VFIO_PCI_DMABUF
 	.attach = vfio_pci_dma_buf_attach,
+#endif
```
Setting `.attach = NULL` when `CONFIG_VFIO_PCI_DMABUF` is disabled means DMA-BUF attach will silently succeed (the core code skips the callback if NULL). This is intentional -- BAR mmap DMABUFs don't need P2P attach semantics -- but it's worth noting that this means the DMABUFs created on the mmap path can't be used for DMA by importers when P2P is disabled.

**Default fault return changed**: The old code defaulted to `VM_FAULT_FALLBACK`; the new code defaults to `VM_FAULT_SIGBUS`. This is correct since a revoked buffer should SIGBUS, and the fallback case is now explicit in the `-EAGAIN` return from `find_pfn`.

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