mcast: do not check 'rv' twice in a row

With the loop, don't check 'rv' twice in a row. Without the loop, 'rv'
doesn't even need to be checked.

Make the comment more grammar-friendly.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>