mm: document and polish read-ahead code

Add some "big-picture" documentation for read-ahead and polish the code
to make it fit this documentation.

The meaning of ->async_size is clarified to match its name.  i.e.  Any
request to ->readahead() has a sync part and an async part.  The caller
will wait for the sync pages to complete, but will not wait for the
async pages.  The first async page is still marked PG_readahead

Note that the current function names page_cache_sync_ra() and
page_cache_async_ra() are misleading.  All ra request are partly sync
and partly async, so either part can be empty.  A page_cache_sync_ra()
request will usually set ->async_size non-zero, implying it is not all
synchronous.

When a non-zero req_count is passed to page_cache_async_ra(), the
implication is that some prefix of the request is synchronous, though
the calculation made there is incorrect - I haven't tried to fix it.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: NeilBrown <[email protected]>
Cc: Anna Schumaker <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Ilya Dryomov <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Lars Ellenberg <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: Paolo Valente <[email protected]>
Cc: Philipp Reisner <[email protected]>
Cc: Ryusuke Konishi <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Wu Fengguang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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