Previously, you'd occasionally see this sort of construct (simplified):
```
return FooDevice::all_instances().with([&](auto& list) {
for (auto& device : list) {
if (/* search condition */)
continue;
return do_something_that_reacquires(device.index());
}
return ENOENT;
});
```
This would only work if the relevant `all_instances()` used a recursive
spinlock, since reacquiring the device would lock that again.
To get around this, a new helper has been added in `Device` through
which devices can be accessed whilst locking the device list, removing
the need to loop through the device type's `all_instances()`.
Previously, the VFS layer would try to handle renames more-or-less by
itself, which really only worked for ext2, and even that was only due to
the replace_child kludge existing specifically for this purpose. This
never worked properly for FATFS, since the VFS layer effectively
depended on filesystems having some kind of reference-counting for
inodes, which is something that simply doesn't exist on any FAT variant
we support.
To resolve various issues with the existing scheme, this commit makes
filesystem implementations themselves responsible for the actual rename
operation, while keeping all the existing validation inside the VFS
layer. The only intended behavior change here is that rename operations
should actually properly work on FATFS.
Similarly to DevPtsFS, this filesystem is about exposing loop device
nodes easily in /dev/loop, so userspace doesn't need to do anything in
order to use new devices immediately.
