ovl: make symbol 'ovl_aops' static

Fixes the following sparse warning:

fs/overlayfs/inode.c:507:39: warning:
 symbol 'ovl_aops' was not declared. Should it be static?

Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from overlayfs over xfs")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -504,7 +504,7 @@ static const struct inode_operations ovl_special_inode_operations = {
 	.update_time	= ovl_update_time,
 };
 
-const struct address_space_operations ovl_aops = {
+static const struct address_space_operations ovl_aops = {
 	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
 	.direct_IO		= noop_direct_IO,
 };