Don't cast away const

Follow up to 511fce6b which missed a cast.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9533

include/sys/range_tree.h

@@ -168,7 +168,7 @@ rs_get_fill_raw(const range_seg_t *rs, const range_tree_t *rt)
 		return (r64->rs_end - r64->rs_start);
 	}
 	case RANGE_SEG_GAP:
-		return (((range_seg_gap_t *)rs)->rs_fill);
+		return (((const range_seg_gap_t *)rs)->rs_fill);
 	default:
 		VERIFY(0);
 		return (0);