zfs_vnops_os.c: fallocate is valid but not supported on FreeBSD

This works around
/usr/lib/go-1.18/pkg/tool/linux_amd64/link:
mapping output file failed: invalid argument

It's happened to me under a Linux jail, but it's also happened to other
people, see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270247#c4

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: pstef <pstef@users.noreply.github.com>
Closes #16918