blackfin: restore L1 base address and length

Restore L1 base address and length to 0 after free else the value will be
wrong.
Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>

arch/blackfin/include/asm/mmu_context.h

@@ -30,8 +30,11 @@ extern void *l1sram_alloc_max(void*);
 static inline void free_l1stack(void)
 {
 	nr_l1stack_tasks--;
-	if (nr_l1stack_tasks == 0)
+	if (nr_l1stack_tasks == 0) {
 		l1sram_free(l1_stack_base);
+		l1_stack_base = NULL;
+		l1_stack_len = 0;
+	}
 }
 
 static inline unsigned long