ip6mr: remove stray rcu_read_unlock() from ip6_mr_forward()

One rcu_read_unlock() should have been removed in blamed commit.

Fixes: 9b1c21d8

 ("ip6mr: do not acquire mrt_lock while calling ip6_mr_forward()")
Reported-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220725200554.2563581-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>