fuse: conditionally fill kstat in fuse_do_statx()

The code path

fuse_update_attributes
    fuse_update_get_attr
        fuse_do_statx

has the risk to use a NULL pointer for struct kstat *stat, although current
callers of fuse_update_attributes() only set request_mask to values that
will trigger the call of fuse_do_getattr(), which already handles the NULL
pointer.  Future updates might miss that fuse_do_statx() does not handle it
it is safer to add a condition already right now.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Fixes: d3045530 ("fuse: implement statx")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>