mm: page cache: store only head pages in i_pages

mm: page cache: store only head pages in i_pages

Transparent Huge Pages are currently stored in i_pages as pointers to
consecutive subpages.  This patch changes that to storing consecutive
pointers to the head page in preparation for storing huge pages more
efficiently in i_pages.

Large parts of this are "inspired" by Kirill's patch
https://lore.kernel.org/lkml/20170126115819[email protected]/

Kirill and Huang Ying contributed several fixes.

[[email protected]: use compound_nr, squish uninit-var warning]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Acked-by: Jan Kara <[email protected]>
Reviewed-by: Kirill Shutemov <[email protected]>
Reviewed-by: Song Liu <[email protected]>
Tested-by: Song Liu <[email protected]>
Tested-by: William Kucharski <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Tested-by: Qian Cai <[email protected]>
Tested-by: Mikhail Gavrilov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Song Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>