For file systems that do not implement holes, Linux provides the trivial implementation
for FIOSEEKDATA/FIOSEEKHOLE that returns the offset for FIOSEEKDATA and offset == file_size
Also, this seemed to be the preferred choice when discussed on freebsd-current@ w.r.t. what
to do when file systems don't implement VOP_IOCTL().
This patch implements this by adding a vop_stdioctl() that does it.