Only respond to the PCIe Attention Button if a device is already plugged in.

Prior to this commit, if PCIEM_SLOT_STA_ABP and PCIEM_SLOT_STA_PDC are
asserted simultaneously, FreeBSD sets a 5 second "hardware going away" timer
and then processes the "presence detect" change. In the (physically
challenging) case that someone presses the "attention button" and inserts
a new PCIe device at exactly the same moment, this results in FreeBSD
recognizing that the device is present, attaching it, and then detaching it
5 seconds later.

On EC2 "bare metal" hardware this is the precise sequence of events which
takes place when a new EBS volume is attached; virtual machines have no
difficulty effecting physically implausible simultaneity.

This patch changes the handling of PCIEM_SLOT_STA_ABP to only detach a
device if the presence of a device was detected *before* the interrupt
which reports the Attention Button push.

Reported by:	Matt Wilson
Reviewed by:	jhb
MFC after:	1 week
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D20499

sys/dev/pci/pci_pci.c

@@ -1170,9 +1170,11 @@ pcib_pcie_intr_hotplug(void *arg)
 {
 	struct pcib_softc *sc;
 	device_t dev;
+	uint16_t old_slot_sta;
 
 	sc = arg;
 	dev = sc->dev;
+	old_slot_sta = sc->pcie_slot_sta;
 	sc->pcie_slot_sta = pcie_read_config(dev, PCIER_SLOT_STA, 2);
 
 	/* Clear the events just reported. */
@@ -1188,7 +1190,8 @@ pcib_pcie_intr_hotplug(void *arg)
 			    "Attention Button Pressed: Detach Cancelled\n");
 			sc->flags &= ~PCIB_DETACH_PENDING;
 			callout_stop(&sc->pcie_ab_timer);
-		} else {
+		} else if (old_slot_sta & PCIEM_SLOT_STA_PDS) {
+			/* Only initiate detach sequence if device present. */
 			device_printf(dev,
 		    "Attention Button Pressed: Detaching in 5 seconds\n");
 			sc->flags |= PCIB_DETACH_PENDING;