Revert "readahead: properly shorten readahead when falling back to do_page_cache_ra()"

In the Linux kernel, the following vulnerability has been resolved:

Revert "readahead: properly shorten readahead when falling back to do_page_cache_ra()"

This reverts commit 7c877586da3178974a8a94577b6045a48377ff25.

Anders and Philippe have reported that recent kernels occasionally hang
when used with NFS in readahead code. The problem has been bisected to
7c877586da3 ("readahead: properly shorten readahead when falling back to
do_page_cache_ra()"). The cause of the problem is that ra->size can be
shrunk by read_pages() call and subsequently we end up calling
do_page_cache_ra() with negative (read huge positive) number of pages.
Let's revert 7c877586da3 for now until we can find a proper way how the
logic in read_pages() and page_cache_ra_order() can coexist. This can
lead to reduced readahead throughput due to readahead window confusion but
that's better than outright hangs.