librbd/migration/HttpClient: socket isn't shut down on some state transitions

librbd/migration/HttpClient: socket isn't shut down on some state transitions

If shut_down() gets delayed until a) the state transition from
STATE_RESET_CONNECTING completes and the reconnect is unsuccessful or
b) the state transition from STATE_RESET_DISCONNECTING completes (i.e.
next_state is STATE_UNINITIALIZED or STATE_RESET_CONNECTING), the
socket needs to be shut down before m_on_shutdown is invoked.  The line
of thought here is the same as for the corresponding state transitions
that don't involve STATE_SHUTTING_DOWN.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>