ceph: ignore the truncate when size won't change with Fx caps issued

If the new size is the same as the current size, the MDS will do nothing
but change the mtime/atime. POSIX doesn't mandate that the filesystems
must update them in this case, so just ignore it instead.

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