in vfs_detach(). vfs_done may free global filesystem's resources,
typically those allocated in respective filesystem's init function.
Needed so those filesystems which went in via LKM have a chance to
clean after themselves before unloading.
For each leaf filesystem, add appropriate vfs_done routine.
Also remember how many times ffs_init() was called and do
the appropriate initialization on first call only. In ffs_done(),
destroy the resources when called by the last user of ffs code.
Change mfs to call ffs_init()/ffs_done() appropriately.
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