Kernel/PCI: Don't hold spinlocks when doing fast device enumeration

Instead, hold the lock while we copy the contents to a stack-based
Vector then iterate on it without any locking.

Because we rely on heap allocations, we need to propagate errors back
in case of OOM condition, therefore, both PCI::enumerate API function
and PCI::Access::add_host_controller_and_enumerate_attached_devices use
now a ErrorOr<void> return value to propagate errors. OOM Error can only
occur when enumerating the m_device_identifiers vector under a spinlock
and trying to expand the temporary Vector which will be used locklessly
to actually iterate over the PCI::DeviceIdentifiers objects.

Kernel/Bus/PCI/Initializer.cpp

@@ -62,9 +62,9 @@ UNMAP_AFTER_INIT void initialize()
 
     PCI::PCIBusSysFSDirectory::initialize();
 
-    PCI::enumerate([&](DeviceIdentifier const& device_identifier) {
+    MUST(PCI::enumerate([&](DeviceIdentifier const& device_identifier) {
         dmesgln("{} {}", device_identifier.address(), device_identifier.hardware_id());
-    });
+    }));
 }
 
 UNMAP_AFTER_INIT bool test_pci_io()