[MIPS] Export empty_zero_page for sake of the ext4 module.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/init.c

@@ -71,6 +71,7 @@ DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
  * don't have to care about aliases on other CPUs.
  */
 unsigned long empty_zero_page, zero_page_mask;
+EXPORT_SYMBOL_GPL(empty_zero_page);
 
 /*
  * Not static inline because used by IP27 special magic initialization code