vfio/type1: check pfn valid before converting to struct page

Check physical PFN is valid before converting the PFN to a struct page
pointer to be returned to caller of vfio_pin_pages().

vfio_pin_pages() pins user pages with contiguous IOVA.
If the IOVA of a user page to be pinned belongs to vma of vm_flags
VM_PFNMAP, pin_user_pages_remote() will return -EFAULT without returning
struct page address for this PFN. This is because usually this kind of PFN
(e.g. MMIO PFN) has no valid struct page address associated.
Upon this error, vaddr_get_pfns() will obtain the physical PFN directly.

While previously vfio_pin_pages() returns to caller PFN arrays directly,
after commit
34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()"),
PFNs will be converted to "struct page *" unconditionally and therefore
the returned "struct page *" array may contain invalid struct page
addresses.

Given current in-tree users of vfio_pin_pages() only expect "struct page *
returned, check PFN validity and return -EINVAL to let the caller be
aware of IOVAs to be pinned containing PFN not able to be returned in
"struct page *" array. So that, the caller will not consume the returned
pointer (e.g. test PageReserved()) and avoid error like "supervisor read
access in kernel mode".

Fixes: 34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()")
Cc: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230519065843.10653-1-yan.y.zhao@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

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