Use hardware timer to simulate interrupt moderation.

Use hardware timer to simulate interrupt moderation.

hw.nfeX.imtimer -> 0  no interrupt moderation at all.
hw.nfeX.imtimer -> -Y hardware timer simulated interrupt moderation,
                      the simultated interrupt moderation timer will
                      be set to Y.
hw.nfeX.imtimer -> Y  hardware timer only interrupt moderation,
                      hardware timer will be set to Y.

Set hw.nfeX.imtimer to -125 (8000Hz) should work on most system.

With heavier interrupt moderation (e.g. hw.nfeX.imtimer to -250), it is
recommended to increase hw.nfe.rx_ring_count tunable to a higher value
(e.g. 512), since the default value (128) is obviously too small.