NFSD: Eliminate percpu counter contention in IO byte accounting

nfsd_stats_io_write_add() and nfsd_stats_io_read_add() accumulate
byte counts in per-net-namespace and per-export percpu_counters
using percpu_counter_add(), which applies the default batch
threshold of max(32, 2*nr_cpus).

For a 4 KB NFS WRITE, the amount (4096) always exceeds this
threshold, so percpu_counter_add_batch() acquires the counter's
global spinlock on every update. Each WRITE RPC updates two
counters (per-net and per-export), producing two global lock
acquisitions per operation. Profiling on a 10-CPU RDMA NFS
server shows 0.44% of total CPU cycles spent contending on
these locks during a small random write workload.

Switch to percpu_counter_add_local(), which batches with
INT_MAX so that updates always remain on the per-CPU fast
path regardless of the amount. All readers of these counters
already use 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/20260717001232.438792-5-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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