xdp-utils: check if alternate doc portal path matches in path_for_fd()

The document portal uses different inode number when exposing a
particular document in different parts of the file system.  As sandboxed
apps only have a subtree of the document portal file system mounted, the
"same file" checks in xdp_app_info_get_path_for_fd() would fail for
document portal paths.

To fix this, we check to see whether the corresponding "by-app/$app_id"
path matches the stat information of the file descriptor.

Fixes #545