Fix CONFIG_GCC_PLUGIN_RANDSTRUCT build

Fix build errors with gcc 7.3.0 on Gentoo with kernel 4.16.3
built with CONFIG_GCC_PLUGIN_RANDSTRUCT=y such as:

module/zfs/vdev_indirect.c:296:2: error:
positional initialization of field in ‘struct’ declared with
‘designated_init’ attribute [-Werror=designated-init]
  vdev_indirect_map_free,
  ^~~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Mark Wright <gienah@gentoo.org>
Closes #7464

module/zfs/vdev_indirect.c

@@ -293,8 +293,8 @@ vdev_indirect_map_free(zio_t *zio)
 }
 
 static const zio_vsd_ops_t vdev_indirect_vsd_ops = {
-	vdev_indirect_map_free,
-	zio_vsd_default_cksum_report
+	.vsd_free = vdev_indirect_map_free,
+	.vsd_cksum_report = zio_vsd_default_cksum_report
 };
 
 /*