rds: tcp: Take explicit refcounts on struct net

rds: tcp: Take explicit refcounts on struct net

It is incorrect for the rds_connection to piggyback on the
sock_net() refcount for the netns because this gives rise to
a chicken-and-egg problem during rds_conn_destroy. Instead explicitly
take a ref on the net, and hold the netns down till the connection
tear-down is complete.

Orabug: 26289770

(Cherry-pick of upstream 8edc3affc0770886c7bfb3436b0fdd09bce13167)

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>