ceph: zero the dir_entries memory when allocating it

This potentially will cause a bug in future if using an old ceph
version that sends a smaller inode struct, which can cause some members
to be skipped in handle_reply.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>