IB/cm: Use separate agent w/o flow control for REP

Most responses (e.g., RTU) are not subject to flow control, as there is
no further response expected.  However, REPs are both requests (waiting
for RTUs) and responses (being waited by REQs).

With agent-level flow control added to the MAD layer, REPs can get
delayed by outstanding REQs.  This can cause a problem in a scenario
such as 2 hosts connecting to each other at the same time.  Both hosts
fill the flow control outstanding slots with REQs.  The corresponding
REPs are now blocked behind those REQs, and neither side can make
progress until REQs time out.

Add a separate MAD agent which is only used to send REPs.  This agent
does not have a recv_handler as it doesn't process responses nor does it
register to receive requests.  Disable flow control for agents w/o a
recv_handler, as they aren't waiting for responses.  This allows the
newly added REP agent to send even when clients are slow to generate
RTU, which would be needed to unblock flow control outstanding slots.

Relax check in ib_post_send_mad to allow retries for this agent.  REPs
will be retried by the MAD layer until CM layer receives a response
(e.g., RTU) on the normal agent and cancels them.

Suggested-by: Sean Hefty <shefty@nvidia.com>
Reviewed-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Sean Hefty <shefty@nvidia.com>
Signed-off-by: Vlad Dumitrescu <vdumitrescu@nvidia.com>
Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Link: https://patch.msgid.link/9ac12d0842b849e2c8537d6e291ee0af9f79855c.1751278420.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>

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