net/rds: Trigger rds_send_ping() more than once

Even though a peer may have already received a
non-zero value for "RDS_EXTHDR_NPATHS" from a node in the past,
the current peer may not.

Therefore it is important to initiate another rds_send_ping()
after a re-connect to any peer:
It is unknown at that time if we're still talking to the same
instance of RDS kernel modules on the other side.

Otherwise, the peer may just operate on a single lane
("c_npaths == 0"), not knowing that more lanes are available.

However, if "c_with_sport_idx" is supported,
we also need to check that the connection we accepted on lane#0
meets the proper source port modulo requirement, as we fan out:

Since the exchange of "RDS_EXTHDR_NPATHS" and "RDS_EXTHDR_SPORT_IDX"
is asynchronous, initially we have no choice but to accept an incoming
connection (via "accept") in the first slot ("cp_index == 0")
for backwards compatibility.

But that very connection may have come from a different lane
with "cp_index != 0", since the peer thought that we already understood
and handled "c_with_sport_idx" properly, as indicated by a previous
exchange before a module was reloaded.

In short:
If a module gets reloaded, we recover from that, but do *not*
allow a downgrade to support fewer lanes.

Downgrades would require us to merge messages from separate lanes,
which is rather tricky with the current RDS design.
Each lane has its own sequence number space and all messages
would need to be re-sequenced as we merge, all while
handling "RDS_FLAG_RETRANSMITTED" and "cp_retrans" properly.

Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20260203055723.1085751-9-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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