ceph: fix race during filling readdir cache

commit af5e5eb5 upstream.

Readdir cache uses page cache to save dentry pointers. When adding
dentry pointers to middle of a page, we need to make sure the page
already exists. Otherwise the beginning part of the page will be
invalid pointers.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Cc: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>