Update checkstyle workflow env to ubuntu-20.04

- `checkstyle` workflow uses ubuntu-20.04 environment
- improved `mancheck.sh` readability

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12713

module/os/freebsd/zfs/vdev_geom.c

@@ -199,7 +199,6 @@ vdev_geom_orphan(struct g_consumer *cp)
 	 * async removal support to invoke a close on this
 	 * vdev once it is safe to do so.
 	 */
-	// cppcheck-suppress All
 	SLIST_FOREACH(elem, priv, elems) {
 		// cppcheck-suppress uninitvar
 		vdev_t *vd = elem->vd;

module/os/linux/spl/spl-generic.c

@@ -657,6 +657,7 @@ hostid_read(uint32_t *hostid)
 		return (error);
 	}
 	size = stat.size;
+	// cppcheck-suppress sizeofwithnumericparameter
 	if (size < sizeof (HW_HOSTID_MASK)) {
 		filp_close(filp, 0);
 		return (EINVAL);