Bilbyfs: C version: Update to Linux kernel after 4.19.
-- use 64-bit timespec
-- changed symlink implementation.
-- remove some compiler warnings
-- fix spelling errors.

[impl] Reorganise impl directory.

We currently have all our filesystem implementation in `impl` directory. This
patch moves all the implementations into `impl/fs` directory. This makes for
better organisation of the source tree when we eventually have `impl/net`,
for network implementations in `Cogent`.