x86/mm/pat: Move the memtype related files to arch/x86/mm/pat/

- pat.c offers, dominantly, the memtype APIs - so rename it to memtype.c.

- pageattr.c is offering, primarily, the set_memory*() page attribute APIs,
which is offered via the <asm/set_memory.h> header: name the .c file
along the same pattern.

I.e. perform these renames, and move them all next to each other in arch/x86/mm/pat/:

pat.c => memtype.c
pat_internal.h => memtype.h
pat_interval.c => memtype_interval.c

pageattr.c => set_memory.c
pageattr-test.c => cpa-test.c

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