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

When we succeed in creating some folios in page_cache_ra_order() but then
need to fallback to single page folios, we don't shorten the amount to
read passed to do_page_cache_ra() by the amount we've already read.  This
then results in reading more and also in placing another readahead mark in
the middle of the readahead window which confuses readahead code.  Fix the
problem by properly reducing number of pages to read.  Unlike previous
attempt at this fix (commit 7c877586) which had to be reverted, we are
now careful to check there is indeed something to read so that we don't
submit negative-sized readahead.

Link: https://lkml.kernel.org/r/20241204181016.15273-3-jack@suse.cz


Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>