Introduce map_writable function

Import map_writable from the ksplice fork project:

       https://github.com/jirislaby/ksplice
Do not use CR0 magic anymore

It's possible to do a writable mappings for the sys_call_tables so we
don't needed to use CR0 method anymore.

Moreover, using stop_machine increases the stability as it's the only
valid method to change the kernel's code on SMP systems.
Remove CR0-related code

Cleanup unused CR0-related stuff
Use tabs for indent and make comment block match the general style
Small style improvements
Merge pull request #39 from alexandernst/feature-stop-machine

Don't use CR0 magic anymore