Without this change, the interrupt test fail with MSI-X environment:

$ sudo ethtool -t enp0s2 offline
[   43.921783] igb 0000:00:02.0: offline testing starting
[   44.855824] igb 0000:00:02.0 enp0s2: igb: enp0s2 NIC Link is Down
[   44.961249] igb 0000:00:02.0 enp0s2: igb: enp0s2 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
[   51.272202] igb 0000:00:02.0: testing shared interrupt
[   56.996975] igb 0000:00:02.0 enp0s2: igb: enp0s2 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
The test result is FAIL
The test extra info:
Register test  (offline)	 0
Eeprom test    (offline)	 0
Interrupt test (offline)	 4
Loopback test  (offline)	 0
Link test   (on/offline)	 0

Here, "4" means an expected interrupt was not delivered.

To fix this, route IRQs correctly to the first MSI-X vector by setting
IVAR_MISC. Also, set bit 0 of EIMS so that the vector will not be
masked. The interrupt test now runs properly with this change:

$ sudo ethtool -t enp0s2 offline
[   42.762985] igb 0000:00:02.0: offline testing starting
[   50.141967] igb 0000:00:02.0: testing shared interrupt
[   56.163957] igb 0000:00:02.0 enp0s2: igb: enp0s2 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
The test result is PASS
The test extra info:
Register test  (offline)	 0
Eeprom test    (offline)	 0
Interrupt test (offline)	 0
Loopback test  (offline)	 0
Link test   (on/offline)	 0

Fixes: 4eefa8f0 ("igb: add single vector msi-x testing to interrupt test")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>