Add hw.dmar.batch_coalesce tunable/sysctl, which specifies rate at

Add hw.dmar.batch_coalesce tunable/sysctl, which specifies rate at
which queued invalidation completion interrupt is requested with
regard to the queued invalidation requests.  In other words, setting
the value of the knob to N requests completion interrupt after N items
are processed.  Existing behaviour is restored by setting
hw.dmar.batch_coalesce=1.

The knob significantly decreases the DMAR qi interrupt rate at the
cost of slightly longer DMAR map entries recycling.

Sponsored by:	The FreeBSD Foundation