tools/vm/page_owner_sort.c: two trivial fixes

1) There is an unused variable. It's better to delete it.
2) One case is missing in the usage().

Link: https://lkml.kernel.org/r/20211213164518.2461-1-hanshenghong2019@email.szu.edu.cn


Signed-off-by: Shenghong Han <hanshenghong2019@email.szu.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>