KVM: MMU: try to fix up page faults before giving up

The vGPU folks would like to trap the first access to a BAR by setting
vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault handler
then can use remap_pfn_range to place some non-reserved pages in the VMA.

This kind of VM_PFNMAP mapping is not handled by KVM, but follow_pfn
and fixup_user_fault together help supporting it.  The patch also supports
VM_MIXEDMAP vmas where the pfns are not reserved and thus subject to
reference counting.

Cc: Xiao Guangrong <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Radim Krčmář <[email protected]>
Tested-by: Neo Jia <[email protected]>
Reported-by: Kirti Wankhede <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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