An implementation of posix_fadvise64().  It adds 368 bytes to my vmlinux and
is worth it.

I didn't bother doing posix_fadvise(), as userspace can implement that by
calling fadvise64().

The main reason for wanting this syscall is to provide userspace with the
ability to explicitly shoot down pagecache when streaming large files.  This
is what O_STEAMING does, only posix_fadvise() is standards-based, and harder
to use.

posix_fadvise() also subsumes sys_readahead().

POSIX_FADV_WILLNEED will generally provide asynchronous readahead semantics
for small amounts of I/O.  As long as things like indirect blocks are aready
in core.

POSIX_FADV_RANDOM gives unprivileged applications a way of disabling
readahead on a per-fd basis, which may provide some benefit for super-seeky
access patterns such as databases.



The POSIX_FADV_* values are already implemented in glibc, and this patch
ensures that they are in sync.

A test app (fadvise.c) is available in ext3 CVS.  See

	http://www.zip.com.au/~akpm/linux/ext3/

for CVS details.

Ulrich has reviewed this patch (thanks).