x86/p2m: fix mismatched unlock

Luckily, due to gfn_unlock() currently mapping to p2m_unlock(), this is
only a cosmetic issue right now.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Despite its cosmetic nature I think it would be better to also fix this
in 4.6.

--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -912,7 +912,7 @@ static int set_typed_p2m_entry(struct do
     omfn = p2m->get_entry(p2m, gfn, &ot, &a, 0, NULL, NULL);
     if ( p2m_is_grant(ot) || p2m_is_foreign(ot) )
     {
-        p2m_unlock(p2m);
+        gfn_unlock(p2m, gfn, 0);
         domain_crash(d);
         return -ENOENT;
     }