ovl: let helper ovl_i_path_real() return the realinode

ovl: let helper ovl_i_path_real() return the realinode

[ Upstream commit b2dd05f107b11966e26fe52a313b418364cf497b ]

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.

There are some changes from upstream commit:
1. Context conflicts caused by 73db6a063c785bc ("ovl: port to
   vfs{g,u}id_t and associated helpers") is handled.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Fixes: ffa5723c6d25 ("ovl: store lower path in ovl_inode")
Cc: <stable@vger.kernel.org> # v5.19
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>