net: rds: drop VLA in rds_for_each_conn_info()

Avoid VLA[1] by using an already allocated buffer passed
by the caller.

[1] https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>