mds: fix race between ms_handle_accept() and ms_handle_reset()

following sequence events may happen.
- client sends reconnect message to a recovering MDS.
- MDS::ms_verify_authorizer() create a session for the client, the
  session is in closed state.
- MDS::handle_client_reconnect() sends session close message to
  the client.
- client closes the old connection and opens a new connection
- MDS::ms_verify_authorizer() is called for a new connection
- MDS::ms_handle_reset() is called for the old connection
- MDS::ms_handle_accept() is called for the new connection

MDS::ms_handle_reset() removes client's session from the session map, this
can cause null pointer dereference when handling client request. The fix
is re-add client's session to the session map in MDS::ms_handle_accept()

Signed-off-by: Yan, Zheng <zyan@redhat.com>