Current readahead logic is broken when a random read pattern is followed by
a long sequential read.  The cause is that on a window miss ra->next_size
is set to ra->average, but ra->average is only updated at the end of a
sequence, so window size will remain 1 until the end of the sequential
read.

This patch fixes this by taking the current sequence length into account
(code taken from towards end of page_cache_readahead()), and also setting
ra->average to a decent value in handle_ra_miss() when sequential access is
detected.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>