[PATCH -next] mm: Mark 'stack_vm_area' with __maybe_unused

[YueHaibing]

Fixes gcc '-Wunused-but-set-variable' warning when
CONFIG_VMAP_STACK is not set:

kernel/fork.c: In function 'dup_task_struct':
kernel/fork.c:843:20: warning:
 variable 'stack_vm_area' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index e2a5156..b38c392 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -840,7 +840,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 {
 	struct task_struct *tsk;
 	unsigned long *stack;
-	struct vm_struct *stack_vm_area;
+	struct vm_struct *stack_vm_area __maybe_unused;
 	int err;
 
 	if (node == NUMA_NO_NODE)