KVM: Drop extra GUP (via check_user_page_hwpoison()) to detect poisoned page

Remove check_user_page_hwpoison() as it's effectively dead code.  Prior to
commit 234b239bea39 ("kvm: Faults which trigger IO release the mmap_sem"),
hva_to_pfn_slow() wasn't actually a slow path in all cases, i.e. would do
get_user_pages_fast() without ever doing slow GUP with FOLL_HWPOISON.

Now that hva_to_pfn_slow() is a straight shot to get_user_pages_unlocked(),
and unconditionally passes FOLL_HWPOISON, it is impossible for hva_to_pfn()
to get an -errno that needs to be morphed to -EHWPOISON.

There are essentially four cases in KVM:

  - npages == 0, then FOLL_NOWAIT, a.k.a. @async, must be true, and thus
    check_user_page_hwpoison() will not be called
  - npages == 1 || npages == -EHWPOISON, all good
  - npages == -EINTR || npages == -EAGAIN, bail early, all good
  - everything else, including -EFAULT, can go down the vma_lookup() path,
    as npages < 0 means KVM went through hva_to_pfn_slow() which passes
    FOLL_HWPOISON

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-16-seanjc@google.com>

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