ipv6: remove redundant store to value after addition

There is no need to store the result of the addition back to variable count
after the addition. The store is redundant, replace += with just +

Cleans up clang scan build warning:
warning: Although the value stored to 'count' is used in the enclosing
expression, the value is never actually read from 'count'

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220628145406.183527-1-colin.i.king@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/route.c

@@ -5934,7 +5934,7 @@ int rt6_dump_route(struct fib6_info *rt, void *p_arg, unsigned int skip)
 		rcu_read_unlock();
 
 		if (err)
-			return count += w.count;
+			return count + w.count;
 	}
 
 	return -1;