Fix definition of BLKGETSIZE64 on FreeBSD

The matching ioctl is DIOCGMEDIASIZE.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Closes #10818