illumos gate

It turns out smb_user_delete (via smb_user_logoff) is racing with smb_session_disconnect, both of which can currently transition the session state to SMB_SESSION_STATE_SHUTDOWN.

When smb_user_delete happens first, it changes the session state to SHUTDOWN without having closed the socket.

In retrospect, my previous change to do that in smb_user_delete was ill-considered,
particularly because it sets that state without closing the session socket.

For the fix, I've added a timeout mechanism for session objects.
When a session has no user objects, we'll have a timeout running, which when it expires will call smb_session_disconnect, which causes the smb_session_receiver thread to destroy the session.

The session timeout is scheduled when:

the session is first created (no users yet)
when last user goes away (user count goes 1 to 0)
The session timeout is cancelled when:

the first user is created (user count goes 0 to 1)
when the reader thread begins termination

Testing: This is tricky to reproduce. We first need to arrange for the existence of an smb_user_t object in state LOGGING_ON and making it stay in that state for a while. The only easy way I know of to do that is run a client under debug and put a breakpoint after the first SMB2_SESSION_SETUP command, leaving the client stopped in the debugger at that point. Check that we have an smb_user_t object in state LOGGING_ON by examining the SMB server state with mdb -k (::smblist).
Once we have a session in state LOGGING_ON, do "svcadm restart network/smb/server"
It might take a couple tries to see the shutdown hang.

Additional testing:
Use the "fksmbd" client under debug so one can easily pause client activity at arbitrary points during session setup and tear-down. I used that for the following test cases:

• Let the client connect, then stop before it sends SMB2 Negotiate.
  Verify (with mdb -k) that the SMB server has a client in state ESTABLISHED
  and that the client session goes away in 30 sec.
• Let the client connect and negotiate, stopping before the first session setup
  Verify that the SMB server has a client in state NEGOTIATED and that
  the client session goes away in 30 sec.
• Let the client connect, negotiate, and send the 1st session setup,
  stopping before the 2nd session setup (the NTLMSSP authenticate)
  Verify that the SMB server has a client session in state NEGOTIATED
  and a user session in state LOGGING_ON. Verify that after 45 sec.
  the that user session goes away. Verify that after 30 sec. more,
  the client session also goes away.
• Let the client connect and authenticate. Verify normal function
  (e.g. the tree connect "shares" commands) Use the "logoff" command
  to drop the user logon. Verify that 30 sec after the SMB logoff happens
  (which the client does about 15 sec. after the "logoff" command)
  the SMB server destroys the client session.

Tested per. description above.