Store a reference to the vnode associated with a file descriptor in the
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