ceph: don't truncate file in atomic_open

commit 7cb99947 upstream.

Clear O_TRUNC from the flags sent in the MDS create request.

`atomic_open' is called before permission check. We should not do any
modification to the file here. The caller will do the truncation
afterward.

Fixes: 124e68e7 ("ceph: file operations")
Signed-off-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
[Xiubo: fixed a trivial conflict for 5.10 backport]
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>