RDS: IB: make fragment size (RDS_FRAG_SIZE) dynamic

RDS: IB: make fragment size (RDS_FRAG_SIZE) dynamic

IB fabric is capable of fragment 4GB of data payload into
send_first, send_middle and send_last. Nevertheless,
RDS fragments each payload into PAGE_SIZE, which is usually
4KB. This patch makes the RDS_FRAG_SIZE for RDS IB transport
dynamic.

In the preperation for subsequent patch(es), this patch
adds per connection peer negotiation to determine the
supported fragment size for IB transport.

Orabug: 21894138
Reviewed-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Signed-off-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>