madvise_willneed() currently has a very strange check on how much readahead
it is prepared to do.

  It is based on the user's rss limit.  But this is usually enormous, and
  the user isn't necessarily going to map all that memory at the same time
  anyway.

  And the logic is wrong - it is comparing rss (which is in bytes) with
  `end - start', which is in pages.

  And it returns -EIO on error, which is not mentioned in the Open Group
  spec and doesn't make sense.


This patch takes it all out and applies the same upper limit as is used in
sys_readahead() - half the inactive list.