xfs: horrifying eBPF hacks

Heh.

So here's a kernel patch that builds on Josef's eBPF return value override patch series to provide an eBPF-kprobe-overridable hook function so that administrators can program XFS to redirect a file to the rt device or the data device depending on their own funny constraints any time that the system is doing the first write into an empty file.

The second patch is against Brendan Gregg's bcc repository; it adds a python script to compile and inject a sample eBPF program that behaves (roughly) the same as Richard's earlier patches.

Soooo... rather than dumping a bunch of static code into XFS to support his particular usecase, we're building him an eBPF Bazooka and telling him to have fun. :P

(More generally, it's a science fair project for letting people customize XFS behavior with eBPF in a controlled manner.)

--D