RDS: IB: use max_mr from HCA caps than max_fmr

All HCA drivers seems to popullate max_mr caps and few of
them do both max_mr and max_fmr.

Hence update RDS code to make use of max_mr.

Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>