librbd/migration/HttpClient: avoid hitting an assert in advance_state()

librbd/migration/HttpClient: avoid hitting an assert in advance_state()

If the shutdown gets delayed until the state transition from
STATE_RESET_CONNECTING completes and the reconnect is successful
(i.e. next_state is STATE_READY), we eventually hit "unexpected
state transition" assert in advance_state().  The reason is that
advance_state() would update m_state and call disconnect() under
STATE_READY instead of STATE_SHUTTING_DOWN.  After the disconnect
maybe_finalize_shutdown() would enter advance_state() again with
STATE_SHUTDOWN as next_state, but the transition to that from
STATE_READY is invalid.

Plug this by not transitioning to next_state if current_state is
STATE_SHUTTING_DOWN.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 1046d610e3d6852258e6c4bf0355d0d13fb197b4)