* Avoid determining the MAC class (LEM/EM or IGB) - possibly even multiple
times - on every interrupt by using an own set of device methods for the
IGB class. This translates to introducing igb_if_intr_{disable,enable}()
and igb_if_{rx,tx}_queue_intr_enable() with that IGB-specific code moved
out of their EM counterparts and otherwise continuing to use the EM IFDI
methods also for IGB.
Note that igb_if_intr_{disable,enable}() also issue E1000_WRITE_FLUSH as
lost with the conversion of igb(4) to iflib(4).
Also note, that the em_if_{disable,enable}_intr() methods are renamed to
em_if_intr_{disable,enable}() for consistency with the names used in the
interface declaration while a it.
* In em_intr():
- Don't bother to bail out if the interrupt type is "legacy", i. e. INTx
or MSI, as iflib(4) doesn't use ift_legacy_intr methods for MSI-X. All
other iflib(4)-based drivers avoid this check, too.
- Given that only the MSI-X interrupts have one-shot behavior (by taking
advantage of the EIAC register), explicitly disable interrupts. Hence,
em_intr() now matches what {em,igb}_irq_fast() previously did (in case
of igb(4) supposedly also to work around MSI message reordering errata
on certain systems).
* In em_if_disable_intr():
- Clear the EIAC register unconditionally for 82574 and not just in case
of MSI-X, matching em_if_enable_intr() and bringing back the last hunk
of r206437 lost with the iflib(4) conversion.
- Write to EM_EIAC for clearing said register instead of to the IGB-only
E1000_EIAC used ever since the iflib(4) conversion.