Move duplicated FS "magic" values to the graphdriver package so they can be shared instead of duplicated

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6172 from tianon/remove-duplicated-magic-values

Move duplicated FS "magic" values to the graphdriver package so they can be shared