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@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
master commit: 1f180822ad
master date: 2015-09-14 13:39:19 +0200

xen/arch/x86/mm/p2m.c

@@ -774,7 +774,7 @@ set_mmio_p2m_entry(struct domain *d, unsigned long gfn, mfn_t mfn)
     omfn = p2m->get_entry(p2m, gfn, &ot, &a, 0, NULL);
     if ( p2m_is_grant(ot) )
     {
-        p2m_unlock(p2m);
+        gfn_unlock(p2m, gfn, 0);
         domain_crash(d);
         return 0;
     }