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>