mm: compaction: move compaction sysctl to its own file

This moves all compaction sysctls to its own file.

Move sysctl to where the functionality truly belongs to improve
readability, reduce merge conflicts, and facilitate maintenance.

I use x86_defconfig and linux-next-20230327 branch
$ make defconfig;make all -jn
CONFIG_COMPACTION=y

add/remove: 1/0 grow/shrink: 1/1 up/down: 350/-256 (94)
Function                                     old     new   delta
vm_compaction                                  -     320    +320
kcompactd_init                               180     210     +30
vm_table                                    2112    1856    -256
Total: Before=21119987, After=21120081, chg +0.00%

Despite the addition of 94 bytes the patch still seems a worthwile
cleanup.

Link: https://lore.kernel.org/lkml/067f7347-ba10-5405-920c-0f5f985c84f4@suse.cz/


Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>