net/rds: set the rds_ib_init_frag based on supported sge

net/rds: set the rds_ib_init_frag based on supported sge

The large fragment size requires the underlying HCA to support N sge. Thus,
this patch sets the rds_ib_init_frag based on the minimal of
(rds_ibdev->max_sge - 1) * PAGE_SIZE and the rds_ib_max_frag module_param.

Orabug: 26770234

Signed-off-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Tested-by: Shih-Yu Huang <shih-yu.huang@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>