staging: erofs: remove the redundant d_rehash() for the root dentry

There is actually no need at all to d_rehash() for the root dentry
as Al pointed out, fix it.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>