mm: meminit: finish initialisation of struct pages before basic setup

mm: meminit: finish initialisation of struct pages before basic setup

Waiman Long reported that 24TB machines hit OOM during basic setup when
struct page initialisation was deferred.  One approach is to initialise
memory on demand but it interferes with page allocator paths.  This patch
creates dedicated threads to initialise memory before basic setup.  It
then blocks on a rw_semaphore until completion as a wait_queue and counter
is overkill.  This may be slower to boot but it's simplier overall and
also gets rid of a section mangling which existed so kswapd could do the
initialisation.

[[email protected]: include rwsem.h, use DECLARE_RWSEM, fix comment, remove unneeded cast]
Signed-off-by: Mel Gorman <[email protected]>
Cc: Waiman Long <[email protected]
Cc: Nathan Zimmer <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Scott Norton <[email protected]>
Tested-by: Daniel J Blueman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>