net/rds: Use the first lane until RDS_EXTHDR_NPATHS arrives

Instead of just blocking the sender until "c_npaths" is known
(it gets updated upon the receipt of a MPRDS PONG message),
simply use the first lane (cp_index#0).

But just using the first lane isn't enough.

As soon as we enqueue messages on a different lane, we'd run the risk
of out-of-order delivery of RDS messages.

Earlier messages enqueued on "cp_index == 0" could be delivered later
than more recent messages enqueued on "cp_index > 0", mostly because of
possible head of line blocking issues causing the first lane to be
slower.

To avoid that, we simply take a snapshot of "cp_next_tx_seq" at the
time we're about to fan-out to more lanes.

Then we delay the transmission of messages enqueued on other lanes
with "cp_index > 0" until cp_index#0 caught up with the delivery of
new messages (from "cp_send_queue") as well as in-flight
messages (from "cp_retrans") that haven't been acknowledged yet
by the receiver.

We also add a new counter "mprds_catchup_tx0_retries" to keep track
of how many times "rds_send_xmit" had to suspend activities,
because it was waiting for the first lane to catch up.

Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20260203055723.1085751-8-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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