Get the vnode from file.f_vnode instead of file.f_data.
Get the vnode from file.f_vnode instead of file.f_data.

Nowadays, f_data points to the vnode only if the underlying filesystem
doesn't use it for other purposes (devfs uses it to store the cdev,
for example).

Found by:	csjp
Reviewed by:	csjp
Approved by:	phk, wes, grehan (mentor)
MFC after:	1 week

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