mount-util: fix fd_is_mount_point() when both the parent and directory are network fs

mount-util: fix fd_is_mount_point() when both the parent and directory are network fs

The second call to name_to_handle_at_loop() didn't check for the specific
errors that can happen when the parent dir is mounted by nfs and instead of
falling back like it's done for the child dir, fd_is_mount_point() failed in
this case.

(cherry picked from commit 964ccab8286a7e75d7e9107f574f5cb23752bd5d)
(cherry picked from commit 8de173ff933510200ac3db77f1ae713f2c4acdc3)