Add a flags parameter to send_cap_msg, so we can request expedited
service from the MDS when we know we'll be waiting on the result.

Set that flag in the case of try_flush_caps. The callers of that
function generally wait synchronously on the result, so it's beneficial
to ask the server to expedite it.
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Reviewed-by: NYan, Zheng <zyan@redhat.com>