[ARM] 4839/1: fixes kernel Oops in /dev/mem device driver for memory map with PHYS_OFF

"cat /dev/mem" may cause kernel Oops for boards with PHYS_OFFSET != 0
because character device is mapped to addresses starting from zero
and there is no protection against such situation.
Patch just add this.
Signed-off-by: Alexandre Rusev <arusev@ru.mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/mmap.c

@@ -120,6 +120,8 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr,
  */
 int valid_phys_addr_range(unsigned long addr, size_t size)
 {
+	if (addr < PHYS_OFFSET)
+		return 0;
 	if (addr + size > __pa(high_memory))
 		return 0;