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

lib/libspl/include/os/freebsd/sys/mount.h

@@ -35,12 +35,8 @@
 #include <string.h>
 #include <stdlib.h>
 
-/*
- * Some old glibc headers don't define BLKGETSIZE64
- * and we don't want to require the kernel headers
- */
 #if !defined(BLKGETSIZE64)
-#define	BLKGETSIZE64		_IOR(0x12, 114, size_t)
+#define	BLKGETSIZE64		DIOCGMEDIASIZE
 #endif
 
 /*