kernel/fork.c: mark 'stack_vm_area' with __maybe_unused

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]

Link: http://lkml.kernel.org/r/1545965190-2381-1-git-send-email-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index c979605fe806..d439c48ecf18 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -841,7 +841,7 @@ 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)