nios2: implement flush_dcache_mmap_lock/unlock

Use spin_lock/unlock_irq instead of doing nothing. This fixes corruptions
of the vma_interval_tree causing the kernel to be stuck in an
infinite loop in vma_interval_tree_foreach.
Signed-off-by: Julien Beraud <julien.beraud@spectracom.orolia.com>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/include/asm/cacheflush.h

@@ -46,7 +46,9 @@ extern void copy_from_user_page(struct vm_area_struct *vma, struct page *page,
 extern void flush_dcache_range(unsigned long start, unsigned long end);
 extern void invalidate_dcache_range(unsigned long start, unsigned long end);
 
-#define flush_dcache_mmap_lock(mapping)		do { } while (0)
-#define flush_dcache_mmap_unlock(mapping)	do { } while (0)
+#define flush_dcache_mmap_lock(mapping) \
+	spin_lock_irq(&(mapping)->tree_lock)
+#define flush_dcache_mmap_unlock(mapping) \
+	spin_unlock_irq(&(mapping)->tree_lock)
 
 #endif /* _ASM_NIOS2_CACHEFLUSH_H */