blktests: changing rdma default driver from rdma_rxe to siw

[Shinichiro Kawasaki]

Hello blktests users,

When you run blktests test "nvme" group with nvme_trtype=rdma, or "srp" (scsi
rdma protocol) test group, rdma_rxe is used as the default driver for rdma
transport. Recently, some test case failures were observed with the rdma_rxe
driver [1], and it is being suggested to change the default driver from
rdma_rxe to siw, which is more stable.

A new GitHub pull request is created for this change and now under review [2].
If you don't care the rdma transport driver choice, this change will not affect
your test runs of "nvme" and "srp" groups. If you care the driver choice, please
be aware of the suggested change. After the change, the config option "use_siw"
will be obsolete and "use_rxe" config option will be introduced.

[1] https://lore.kernel.org/linux-rdma/8be8f611-e413-9584-7c2e-2c1abf4147be@acm.org/
[2] https://github.com/osandov/blktests/pull/125