ovl: let helper ovl_i_path_real() return the realinode

Let helper ovl_i_path_real() return the realinode to prepare for
checking non-null realinode in RCU walking path.

[msz] Use d_inode_rcu() since we are depending on the consitency
between dentry and inode being non-NULL in an RCU setting.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Fixes: ffa5723c ("ovl: store lower path in ovl_inode")
Cc: <stable@vger.kernel.org> # v5.19
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>