mm/compaction: move compaction_suitable's comment to right place

Since commit 837d026d ("mm/compaction: more trace to understand
when/why compaction start/finish"), the comment place is not suitable.

So move compaction_suitable's comment to right place.

Link: https://lkml.kernel.org/r/20201116144121.GA385717@rlk


Signed-off-by: Hui Su <sh_def@163.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>