x86: Call HVMOP_pagetable_dying on exit_mmap.

When a pagetable is about to be destroyed, we notify Xen so that the
hypervisor can clear the related shadow pagetable.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/mmu.h

@@ -60,4 +60,5 @@ void  xen_ptep_modify_prot_commit(struct mm_struct *mm, unsigned long addr,
 unsigned long xen_read_cr2_direct(void);
 
 extern void xen_init_mmu_ops(void);
+extern void xen_hvm_init_mmu_ops(void);
 #endif	/* _XEN_MMU_H */