ipv6: Shrink udp_v6_mcast_next() to one socket variable

To avoid the confusion of having two variables, shrink the function to
only use the parameter variable for looping.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: David S. Miller <davem@davemloft.net>