x86/mem_sharing: add missing m2p entry when mapping shared_info page

When mapping in the shared_info page to a fork the m2p entry wasn't set
resulting in the shared_info being reset even when the fork reset was called
with only reset_state and not reset_memory. This results in an extra
unnecessary TLB flush.

Fixes: 1a0000ac77 ("mem_sharing: map shared_info page to same gfn during fork")
Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/mm/mem_sharing.c

@@ -1847,6 +1847,8 @@ static int copy_special_pages(struct domain *cd, struct domain *d)
                                 p2m_ram_rw, p2m->default_access, -1);
             if ( rc )
                 return rc;
+
+            set_gpfn_from_mfn(mfn_x(new_mfn), gfn_x(old_gfn));
         }
     }