how to assure that the selected filesystem hasn't been mounted in the mount operation

[neversetsun at ustc.edu (时正)]

Hi,
I can't find any code to prevent a new filesystem be mounted twice in 
mount().
In mount(), Is_node_fs_root() is checked. But this only means that "you 
can only mount
 one file system onto a single mount point" as the comment says.

I think that rtems_filesystem_mount_table_control maybe traversed again 
to assure fs_ops pointer
has been in the chain.

best regard, shizheng