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.cnSigned-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>

tools/vm/page_owner_sort.c

@@ -41,8 +41,6 @@ static struct block_list *list;
 static int list_size;
 static int max_size;
 
-struct block_list *block_head;
-
 int read_block(char *buf, int buf_size, FILE *fin)
 {
 	char *curr = buf, *const buf_end = buf + buf_size;
@@ -249,7 +247,8 @@ static void usage(void)
 		"-p	Sort by pid.\n"
 		"-a	Sort by memory allocate time.\n"
 		"-r	Sort by memory release time.\n"
-		"-c	cull by comparing stacktrace instead of total block.\n"
+		"-c	Cull by comparing stacktrace instead of total block.\n"
+		"-f	Filter out the information of blocks whose memory has not been released.\n"
 	);
 }