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 <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: pstef <[email protected]>
Closes openzfs#16918

Author: pstef

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -6258,7 +6258,7 @@ struct vop_vector zfs_vnodeops = {
 	.vop_fplookup_vexec = zfs_freebsd_fplookup_vexec,
 	.vop_fplookup_symlink = zfs_freebsd_fplookup_symlink,
 	.vop_access =		zfs_freebsd_access,
-	.vop_allocate =		VOP_EINVAL,
+	.vop_allocate =		VOP_EOPNOTSUPP,
 #if __FreeBSD_version >= 1400032
 	.vop_deallocate =	zfs_deallocate,
 #endif