FreeBSD: get_zfs_ioctl_version() should be cast to (void)

FreeBSD's Coverity scans complain that we ignore the return value. There
is no need to check the return value so we cast it to (void) to suppress
further complaints by static analyzers.

Reported-by: Coverity (CID 1018175)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: szubersk <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14136