ovl: use cached acl on underlying layer

Instead of calling ->get_acl() directly, use get_acl() to get the cached
value.

We will have the acl cached on the underlying inode anyway, because we do
permission checking on the both the overlay and the underlying fs.

So, since we already have double caching, this improves performance without
any cost.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -10,6 +10,7 @@
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/xattr.h>
+#include <linux/posix_acl.h>
 #include "overlayfs.h"
 
 static int ovl_copy_up_truncate(struct dentry *dentry)
@@ -314,14 +315,14 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type)
 	const struct cred *old_cred;
 	struct posix_acl *acl;
 
-	if (!IS_POSIXACL(realinode))
+	if (!IS_ENABLED(CONFIG_FS_POSIX_ACL) || !IS_POSIXACL(realinode))
 		return NULL;
 
 	if (!realinode->i_op->get_acl)
 		return NULL;
 
 	old_cred = ovl_override_creds(inode->i_sb);
-	acl = realinode->i_op->get_acl(realinode, type);
+	acl = get_acl(realinode, type);
 	revert_creds(old_cred);
 
 	return acl;