NFSD: Eliminate percpu counter contention in DRC memory accounting

The DRC memory usage counter (NFSD_STATS_DRC_MEM_USAGE) tracks
bytes, but percpu_counter_add() uses the global percpu_counter_batch
threshold of max(32, 2*nr_cpus). Each DRC entry add or removal
updates the counter by sizeof(struct nfsd_cacherep) (~144 bytes),
which always exceeds the batch threshold. percpu_counter_add()
then acquires the counter's global spinlock on every update,
serializing all nfsd threads.

On a 10-CPU NFS server handling a high rate of non-idempotent
NFSv3 operations, this lock accounts for a measurable fraction
of total spin lock overhead because nfsd_cache_lookup() both
inserts a new entry and prunes up to three old entries per RPC,
producing 4-7 global lock acquisitions per operation.

Switch to percpu_counter_add_local() and percpu_counter_sub_local(),
which batch with INT_MAX so that updates always remain on the per-CPU
fast path regardless of the amount. The only reader of this counter uses
percpu_counter_sum_positive(), which sums the per-CPU deltas under the
global lock, so read accuracy is unaffected.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260717001232.438792-3-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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