linux_file structure and use it instead of directly accessing td_fpop
when destroying the linux_file structure. The td_fpop pointer is not
valid when a cdevpriv destructor is run, and the type-specific close
method has already been called, so f_vnode may not be valid (and the
vnode might have been recycled without our own reference).
Tested by: Julian Stecklina <jsteckli@os.inf.tu-dresden.de>
MFC after: 1 week
