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/[email protected]
Fixes: 901a269ff3 ("filemap: fix page_cache_next_miss() when no hole found")
Signed-off-by: Chi Zhiling <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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;