A new local variable is introduced for accessing p2m entry with lock protection.

Cc: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Hillf Danton <hdanton@sina.com>
---

--- a/arch/arm/xen/p2m.c	2019-04-30 12:32:05.363768200 +0800
+++ b/arch/arm/xen/p2m.c	2019-04-30 12:58:19.854334100 +0800
@@ -70,8 +70,9 @@ unsigned long __pfn_to_mfn(unsigned long
		entry = rb_entry(n, struct xen_p2m_entry, rbnode_phys);
		if (entry->pfn <= pfn &&
				entry->pfn + entry->nr_pages > pfn) {
+			unsigned long mfn = entry->mfn + (pfn - entry->pfn);
			read_unlock_irqrestore(&p2m_lock, irqflags);
-			return entry->mfn + (pfn - entry->pfn);
+			return mfn;
		}
		if (pfn < entry->pfn)
			n = n->rb_left;
--