mm/memory.c: Add memory read privilege on page fault handling

Here add pte_sw_mkyoung function to make page readable on MIPS
platform during page fault handling. This patch improves page
fault latency about 10% on my MIPS machine with lmbench
lat_pagefault case.

It is noop function on other arches, there is no negative
influence on those architectures.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/pgtable.h

@@ -414,6 +414,8 @@ static inline pte_t pte_mkyoung(pte_t pte)
 	return pte;
 }
 
+#define pte_sw_mkyoung	pte_mkyoung
+
 #ifdef CONFIG_MIPS_HUGE_TLB_SUPPORT
 static inline int pte_huge(pte_t pte)	{ return pte_val(pte) & _PAGE_HUGE; }