The POSIX_FADV_NOREUSE hint means "the application will use this range of the
file a single time".  It seems to be intended that the implementation will use
this hint to perform drop-behind of that part of the file when the application
gets around to reading or writing it.

However for reasons which aren't obvious (or sane?) I mapped
POSIX_FADV_NOREUSE onto POSIX_FADV_WILLNEED.  ie: it does readahead.

That's daft.  So for now, make POSIX_FADV_NOREUSE a no-op.

This is a non-back-compatible change.  If someone was using POSIX_FADV_NOREUSE
to perform readahead, they lose.  The likelihood is low.

If/when we later implement POSIX_FADV_NOREUSE things will get interesting - to
do it fully we'll need to maintain file offset/length ranges and peform all
sorts of complex tricks, and managing the lifetime of those ranges' data
structures will be interesting..

A sensible implementation would probably ignore the file range and would
simply mark the entire file as needing some form of drop-behind treatment.

Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>