vfs: skip inodes without pages to free in drop_pagecache_sb()

Many inodes have no pagecache, so we can avoid lots of lock-takings.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Fengguang Wu <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>