Improvements to miscdevice handling

[alexforencich]

When working on the Corundum device driver, I noticed a few things in the exanic kernel module that can be improved.

First, misc_dev.parent should be set to the device object before registering it. If you do that, then symlinks will be created in sysfs to cross-link the miscdevice and the PCIe device. This change is quite simple:

     exanic->misc_dev.name = exanic->name;
     exanic->misc_dev.fops = &exanic_fops;
+    exanic->misc_dev.parent = dev;
     err = misc_register(&exanic->misc_dev);

(looks like this needs to be done in two places, as there are two calls to misc_register)

Second, you can update exanic_get_sysfs_path to use that symlink to find the PCIe device from the miscdevice instead of one of the network interfaces by doing realpath on "/sys/class/misc/%s/device" with exanic->name. This could enable things like hot reset to work during firmware updates even if no network interfaces are registered.

Third, miscdevice already handles the find_by_minor stuff internally, and by default private_data points to the miscdevice struct. All you need to do to get a reference to the exanic struct is something like this in open, release, mmap, and ioctl:

struct miscdevice *miscdev = filp->private_data;
struct exanic *exanic = container_of(miscdev, struct exanic, misc_dev);

(and don't forget to leave filp->private_data alone in exanic_open)