filemap: fix page_cache_next_miss() when no hole found

page_cache_next_miss() should return value outside of the specified range
when no hole is found.  However currently it will return the last index
*in* the specified range confusing ondemand_readahead() to think there's a
hole in the searched range and upsetting readahead logic.

Link: https://lkml.kernel.org/r/20240625101909.12234-2-jack@suse.czSigned-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>

mm/filemap.c

@@ -1752,12 +1752,12 @@ pgoff_t page_cache_next_miss(struct address_space *mapping,
 	while (max_scan--) {
 		void *entry = xas_next(&xas);
 		if (!entry || xa_is_value(entry))
-			break;
+			return xas.xa_index;
 		if (xas.xa_index == 0)
-			break;
+			return 0;
 	}
 
-	return xas.xa_index;
+	return index + max_scan;
 }
 EXPORT_SYMBOL(page_cache_next_miss);