ovl: fix ptr_ret.cocci warnings

fs/overlayfs/export.c:459:10-16: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: 4b91c30a5a19 ("ovl: lookup connected ancestor of dir in inode cache")
CC: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/export.c

@@ -620,7 +620,7 @@ static struct dentry *ovl_lookup_real(struct super_block *sb,
 			if (err == -ECHILD) {
 				this = ovl_lookup_real_ancestor(sb, real,
 								layer);
-				err = IS_ERR(this) ? PTR_ERR(this) : 0;
+				err = PTR_ERR_OR_ZERO(this);
 			}
 			if (!err) {
 				dput(connected);