ice: Honor iflib transmit completion batching

[kbowling]

Summary

ice marked every transmitted packet RS and recorded every last
descriptor in its report-status queue.  Hardware therefore wrote
descriptor status for every packet, and the driver traversed every
packet while reclaiming completed descriptors.

iflib marks selected packets with IPI_TX_INTR as completion
checkpoints.  It forces a checkpoint as deferred work or ring pressure
grows.  Retain EOP on every packet, but set RS and record the descriptor
only at those checkpoints.

DPDK uses the same sparse-RS design and defaults tx_rs_thresh to 32.
Let iflib choose the adaptive interval for FreeBSD.  This reduces PCIe
and memory traffic while preserving bounded descriptor reclamation.

Validated on an E810-XXV in an A-B-A test with five matched
four-stream, TSO-disabled transmit runs per phase.  Median throughput
was 9.413, 9.413, and 9.414 Gbps.  Median whole-system CPU was 21.54%,
17.75%, and 21.62%, respectively.  The candidate used less system CPU
than both exact-baseline phases in every pair.  Interrupt rate was
unchanged, no transmit watchdog fired, and normal TSO traffic remained
line-rate.  DTrace confirmed that only iflib-selected packet-final
descriptors carried RS after the change.  The effect should be more
profound at 200Gbps but my DUT is network limited.

MFC after:      2 weeks