net/rds: Give each connection path its own workqueue

RDS was written to require ordered workqueues for "cp->cp_wq":
Work is executed in the order scheduled, one item at a time.

If these workqueues are shared across connections,
then work executed on behalf of one connection blocks work
scheduled for a different and unrelated connection.

Luckily we don't need to share these workqueues.
While it obviously makes sense to limit the number of
workers (processes) that ought to be allocated on a system,
a workqueue that doesn't have a rescue worker attached,
has a tiny footprint compared to the connection as a whole:
A workqueue costs ~900 bytes, including the workqueue_struct,
pool_workqueue, workqueue_attrs, wq_node_nr_active and the
node_nr_active flex array.  Each connection can have up to 8
(RDS_MPATH_WORKERS) paths for a worst case of ~7 KBytes per
connection.  While an RDS/IB connection totals only ~5 MBytes.

So we're getting a signficant performance gain
(90% of connections fail over under 3 seconds vs. 40%)
for a less than 0.02% overhead.

RDS doesn't even benefit from the additional rescue workers:
of all the reasons that RDS blocks workers, allocation under
memory pressue is the least of our concerns. And even if RDS
was stalling due to the memory-reclaim process, the work
executed by the rescue workers are highly unlikely to free up
any memory. If anything, they might try to allocate even more.

By giving each connection path its own workqueues, we allow
RDS to better utilize the unbound workers that the system
has available.

Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20260109224843.128076-3-achender@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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