tun: default pin CPUs avoid NIC IRQ cores

When tun.pin_threads is on and tun.cpu_affinity is unset, pick pin CPUs
from the allowed set that do not service any up physical NIC's MSI vectors
(read-only walk of /sys/class/net/*/device/msi_irqs and
/proc/irq/*/effective_affinity_list). The old allowed[i] default pinned
encrypt threads onto exactly the cores drivers affine their first RX queue
IRQs to; a flow whose RSS queue fired there had NAPI fighting encrypt for
the core (measured 8.4 vs 10.2 Gbps REV bimodality). Falls back to the old
spread, with a log, when there aren't enough IRQ-free CPUs - e.g. drivers
that allocate one queue per core until the admin narrows them (ethtool -L).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

util/irqcpus_other.go

@@ -0,0 +1,9 @@
+//go:build !linux || android || e2e_testing
+
+package util
+
+// NICIRQCPUs reports no IRQ information on platforms without the linux
+// sysfs interface; callers fall back to their non-IRQ-aware defaults.
+func NICIRQCPUs() (map[int]bool, error) {
+	return nil, nil
+}