FreeBSD compile fix

The file module/os/freebsd/zfs/zfs_ioctl_compat.c fails compiling
because of this error: 'static' is not at beginning of declaration

This commit fixes the three places within that file.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13702