tools/vm/page_owner_sort.c: sort by stacktrace before culling

tools/vm/page_owner_sort.c: sort by stacktrace before culling

The contents of page_owner have changed to include more information than
the stack trace.  On a modern kernel, the blocks look like

  Page allocated via order 0, mask 0x0(), pid 1, ts 165564237 ns, free_ts 0 ns
    register_early_stack+0x4b/0x90
    init_page_owner+0x39/0x250
    kernel_init_freeable+0x11e/0x242
    kernel_init+0x16/0x130

Sorting by the contents of .txt will result in almost no repeated pages,
as the pid, ts, and free_ts will almost never be the same.  Instead,
sort by the contents of the stack trace, which we assume to be whatever
is after the first line.

[seanga2@gmail.com: fix NULL-pointer dereference when comparing stack traces]
Link: https://lkml.kernel.org/r/20211125162653.1855958-1-seanga2@gmail.com
Link: https://lkml.kernel.org/r/20211124193709.1805776-1-seanga2@gmail.com
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Cc: Changhee Han <ch0.han@lge.com>
Cc: Tang Bin <tangbin@cmss.chinamobile.com>
Cc: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Cc: Zhenliang Wei <weizhenliang@huawei.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Yinan Zhang <zhangyinan2019@email.szu.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>