NFSD: Eliminate percpu counter contention in reply cache statistics

Each RPC passes through nfsd_cache_lookup(), which increments one
of nfsd_stats_rc_hits_inc(), nfsd_stats_rc_misses_inc(), or
nfsd_stats_rc_nocache_inc(). These helpers update
per-net-namespace percpu_counters with percpu_counter_inc(),
which applies the default batch threshold of max(32, 2*nr_cpus).

Once a CPU's local delta reaches that threshold, the update folds
into the shared counter under its global spinlock. On a busy
multi-CPU server this produces lock traffic on a counter cacheline
shared across all CPUs, growing with the request rate.

Switch to percpu_counter_add_local(fbc, 1), which batches with
INT_MAX so that increments always remain on the per-CPU fast path.
This matches the treatment already applied to the IO byte and DRC
memory counters. All readers of these counters 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/20260717001232.438792-4-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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