sunrpc: guarantee a thread per pool when auto-distributing

svc_set_num_threads() spreads the requested thread count evenly across
the service's pools. In pernode mode each pool maps to a NUMA node, and
svc_pool_for_cpu() steers an incoming transport to the pool for the node
it arrived on. When fewer threads than pools are requested, even
distribution leaves some pools empty, and a transport steered to an
empty pool has no thread to service it.

Floor each pool at one thread when auto-distributing a non-zero count,
so no pool is left empty. Every pool maps to a node that had CPUs when
the pool map was built (svc_pool_map_init_pernode() only creates pools
for nodes returned by for_each_node_with_cpus()), so there is no pool
that should be left threadless. The resulting total may exceed the
requested count. This only affects the auto-distribute path (a
single-value array, i.e. svc_set_num_threads()); callers that set
per-pool counts explicitly via svc_set_pool_threads() are unchanged and
may still set a pool to zero.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260706-sunrpc-pool-mode-v5-3-6c4ee7cd89aa@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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