Use BlueFlame for RDMA Write [with immediate] without data

Use Blue-Flame for RDMA Write and RDMA Write with immediate without
any data (no s/g).  This improves latency for those messages.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>