Linux 5.3: Fix switch() fall though compiler errors

Fix some switch() fall-though compiler errors:

    abd.c:1504:9: error: this statement may fall through

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #9170

module/zfs/vdev_raidz_math_scalar.c

@@ -142,6 +142,7 @@ static const struct {
 		a.b[6] = mul_lt[a.b[6]];				\
 		a.b[5] = mul_lt[a.b[5]];				\
 		a.b[4] = mul_lt[a.b[4]];				\
+		/* falls through */					\
 	case 4:								\
 		a.b[3] = mul_lt[a.b[3]];				\
 		a.b[2] = mul_lt[a.b[2]];				\