ceph: properly get capsnap's size in get_oldest_context()

capsnap's size is set by __ceph_finish_cap_snap(). If capsnap is under
writing, its size is zero. In this case, get_oldest_context() should
read i_size. Besides, ceph_writepages_start() should re-check capsnap's
size after dirty pages get locked.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>