Migrate libcfs to emulate Linux kernel APIs, so that when submitting Linux client to upstream kernel, we don't need the abstraction layer which will be rejected by kernel maintainers. The majority of libcfs are cfs_* or ll_* wrappers, and can be cleaned up with scripts doing like "s/cfs_spin_lock/spin_lock". For other functions that cannot be cleaned up this way, we need to look at them in a case-by-case basis.