btrfs: add and use readahead_batch_length

Implement readahead_batch_length() to determine the number of bytes in
the current batch of readahead pages and use it in btrfs. Also use the
readahead_pos to get the offset.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>