RDS: fix IB transport flow control

RDS: fix IB transport flow control

Orabug: 22306628

IB flow control is always disabled regardless of
rds_ib_sysctl_flow_control flag.
The issue is that the initial credit advertisement
annouces zero credits, because ib_recv_refill() has
not yet been called. An initial credit offering
of zero effectively disables flow control.

IB flow control is only enabled if both active and
passive connections have set the rds_ib_sysctl
flow_control flag. E.g,

Conn. A (on),  Conn. B (on)  = enable
Conn. A (off), Conn. B (on)  = disable
Conn. A (on),  Conn. B (off) = disable
Conn. A (off), Conn. B (off) = disable

Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Knut Omang <knut.omang@oracle.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Acked-by: Ajaykumar Hotchandani <ajaykumar.hotchandani@oracle.com>
Signed-off-by: Wei Lin Guay <wei.lin.guay@oracle.com>