Thomas, Roland and me agreed on the interface change.  This needs to be implemented now.  The read side you already implemented.  I will make this task depend on that patch.

Fatfs needs to lock the directory that holds a file in order to write
the metadata of a file to disk. This happens in the function
write_node.

The problem is that this directory could have been locked by one of
the callers of write_node. So locking the directory is not safe
because the directory could have been locked (and that makes the
thread hang). Not locking the directory isn't safe either because it
introduces race conditions.

We can't make a special case here like we did for read_node because
there are too many callers, all with different behavior.

I propose to modify the interfaces for diskfs_update_node and
diskfs_write_disknode. By adding a flag to notice these functions if
the directory is locked, or even better by passing the "dp" to the
function if it is locked (as Roland once proposed for read_node).

I even like to make similar changes for diskfs_cached_lookup so fatfs
and other filesystems that have similar problems can support nfs.