net/rds: fix WARNING in rds_conn_connect_if_down

If connection isn't established yet, get_mr() will fail, trigger connection after
get_mr().

Fixes: 584a8279

 ("RDS: RDMA: return appropriate error on rdma map failures")
Reported-and-tested-by:  <syzbot+d4faee732755bba9838e@syzkaller.appspotmail.com>
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: David S. Miller <davem@davemloft.net>