selftests: rds: report missing RDMA prereqs as XFAIL

selftests: rds: report missing RDMA prereqs as XFAIL

Make the RDMA test return XFAIL rather than skip when RXE is not
available, since the RDMA datapath is not run in netdev CI.

Change the three RDMA-prerequisite checks in check_rdma_conf() and
check_rdma_conf_enabled() to exit with KSFT_XFAIL (2) and tag their
messages [XFAIL] instead of [SKIP].

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260602050657.26389-5-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>