readahead: fix return value of page_cache_next_miss() when no hole is found

max_scan in page_cache_next_miss always decreases to zero when no hole is
found, causing the return value to be index + 0.

Fix this by preserving the max_scan value throughout the loop.

Jan said "From what I know and have seen in the past, wrong responses
from page_cache_next_miss() can lead to readahead window reduction and
thus reduced read speeds."

Link: https://lkml.kernel.org/r/20250605054935.2323451-1-chizhiling@163.com
Fixes: 901a269 ("filemap: fix page_cache_next_miss() when no hole found")
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Chi Zhiling

mm/filemap.c

@@ -1778,8 +1778,9 @@ pgoff_t page_cache_next_miss(struct address_space *mapping,
 			     pgoff_t index, unsigned long max_scan)
 {
 	XA_STATE(xas, &mapping->i_pages, index);
+	unsigned long nr = max_scan;
 
-	while (max_scan--) {
+	while (nr--) {
 		void *entry = xas_next(&xas);
 		if (!entry || xa_is_value(entry))
 			return xas.xa_index;