RDMA/cm: Make QP FLUSHABLE for supported device

RDMA/cm: Make QP FLUSHABLE for supported device

Similar to RDMA and Atomic qp attributes enabled by default in CM, enable
FLUSH attribute for supported device. That makes applications that are
built with rdma_create_ep, rdma_accept APIs have FLUSH qp attribute
natively so that user is able to request FLUSH operation simpler.

Note that, a FLUSH operation requires FLUSH are supported by both
device(HCA) and memory region(MR) and QP at the same time, so it's safe
to enable FLUSH qp attribute by default here.

FLUSH attribute can be disable by modify_qp() interface.

Link: https://lore.kernel.org/r/20221206130201.30986-10-lizhijian@fujitsu.com
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>