mm/page_alloc: make the annotations of available memory more accurate

Not all systems use swap, so estimating available memory would help to
prevent swapping or OOM of system that not use swap.

And we need to reserve some page cache to prevent swapping or thrashing. 
If somebody is accessing the pages in pagecache, and if too much would be
freed, most accesses might mean reading data from disk, i.e.  thrashing.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Yang <[email protected]>
Signed-off-by: CGEL ZTE <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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