tools/vm/page_owner_sort.c: support sorting blocks by multiple keys

When viewing page owner information, we may want to sort blocks of
information by multiple keys, since one single key does not uniquely
identify a block. Therefore, following adjustments are made:

1. Add a new --sort option to support sorting blocks of information by
multiple keys.

	./page_owner_sort <input> <output> --sort=<order>
	./page_owner_sort <input> <output> --sort <order>

<order> is a single argument in the form of a comma-separated list,
which offers a way to specify sorting order.

Sorting syntax is [+|-]key[,[+|-]key[,...]]. The ascending or descending
order can be specified by adding the + (ascending, default) or - (descend
-ing) prefix to the key:

	./page_owner_sort <input> <output> [option] --sort -key1,+key2,key3...

For example, to sort the blocks first by task command name in lexicographic
order and then by pid in ascending numerical order, use the following:

	./page_owner_sort <input> <output> --sort=name,+pid

To sort the blocks first by pid in ascending order and then by timestamp
of the page when it is allocated in descending order, use the following:

	./page_owner_sort <input> <output> --sort=pid,-alloc_ts

2. Add explanations of a newly added --sort option in the function usage()
and the document(Documentation/vm/page_owner.rst).

This work is coauthored by
	Yixuan Cao
	Shenghong Han
	Yinan Zhang
	Chongxi Zhao
	Yuhong Feng
	Yongqiang Liu

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiajian Ye <[email protected]>
Cc: Chongxi Zhao <[email protected]>
Cc: Shenghong Han <[email protected]>
Cc: Yinan Zhang <[email protected]>
Cc: Yixuan Cao <[email protected]>
Cc: Yongqiang Liu <[email protected]>
Cc: Yuhong Feng <[email protected]>
Cc: Haowen Bai <[email protected]>
Cc: Sean Anderson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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