x86/mm/pat: Update the comments in pat.c and pat_interval.c and refresh the code a bit

Tidy up the code:

 - add comments explaining the PAT code, the role of the functions and the logic

 - fix various typos and grammar while at it

 - simplify the file-scope memtype_interval_*() namespace to interval_*()

 - simplify stylistic complications such as unnecessary linebreaks
   or convoluted control flow

 - use the simpler '#ifdef CONFIG_*' pattern instead of '#if defined(CONFIG_*)' pattern

 - remove the non-idiomatic newline between late_initcall() and its function definition

Signed-off-by: Ingo Molnar <mingo@kernel.org>