fs: remove useless plus one in super_cache_scan()

After commit 475d0db7 ("fs: Fix theoretical division by 0 in
super_cache_scan()."), there's no need to plus one to prevent
division by zero.

Remove it to simplify the code.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Link: https://lore.kernel.org/20250428135050.267297-1-alexjlzheng@tencent.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>