bilbyfs[C]: Fix builds with the 5.x series of kernels.

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

fs: Rename inode_change_ok -> settattr_prepare in latest kernels

build: Fix build issues on lemma

Bilby builds will still fail. These are fixes for ext2 only.

fs: More version number issues sorted out.

bilby: Fix build issues for 4.x kernels

This commit, just like the last one, fixes the conditional build
versions for the various versions of the kernel we are building.

bilby: Use the proper kernel version for conditional builds

impl: Fix build issues on Linux

[impl] Get both Bilby and ext2 working

Support to build (and work) with the latest Linux kernel releases.

linux: Replace CURRENT_TIME macro.

`CURRENT_TIME` macro has been deprecated(and replaced) in the Linux
kernel because it isn't safe on 32bit systems. (y2038 problem).
This patch fixes Cogent code and drivers to reflect that.

[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`.