readahead: drop pointless index from force_page_cache_ra()

Current index to readahead is tracked in readahead_control and properly
updated by page_cache_ra_unbounded() (read_pages() in fact).  So there's
no need to track the index separately in force_page_cache_ra().

Link: https://lkml.kernel.org/r/20240625101909.12234-4-jack@suse.cz


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Zhang Peng <zhangpengpeng0808@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>