IB/rxe: increase max MR limit

Increase the max MR limit to support more I/O queues
for NVMe over Fabrics hosts.

Signed-off-by: Vijay Immanuel <vijayi@attalasystems.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>