smb: client: fix race condition in negotiate timeout by using more precise timing

When the SMB server reboots and the client immediately accesses the mount
point, a race condition can occur that causes operations to fail with
"Host is down" error.

Reproduction steps:
  # Mount SMB share
  mount -t cifs //192.168.245.109/TEST /mnt/ -o xxxx
  ls /mnt

  # Reboot server
  ssh root@192.168.245.109 reboot
  ssh root@192.168.245.109 /path/to/cifs_server_setup.sh
  ssh root@192.168.245.109 systemctl stop firewalld

  # Immediate access fails
  ls /mnt
  ls: cannot access '/mnt': Host is down

  # But works if there is a delay

The issue is caused by a race condition between negotiate and reconnect.
The 20-second negotiate timeout mechanism can interfere with the normal
recovery process when both are triggered simultaneously.

  ls                              cifsd
---------------------------------------------------
 cifs_getattr
 cifs_revalidate_dentry
 cifs_get_inode_info
 cifs_get_fattr
 smb2_query_path_info
 smb2_compound_op
 SMB2_open_init
 smb2_reconnect
 cifs_negotiate_protocol
  smb2_negotiate
   cifs_send_recv
    smb_send_rqst
    wait_for_response
                            cifs_demultiplex_thread
                              cifs_read_from_socket
                              cifs_readv_from_socket
                                server_unresponsive
                                cifs_reconnect
                                  __cifs_reconnect
                                  cifs_abort_connection
                                    mid->mid_state = MID_RETRY_NEEDED
                                    cifs_wake_up_task
    cifs_sync_mid_result
     // case MID_RETRY_NEEDED
     rc = -EAGAIN;
   // In smb2_negotiate()
   rc = -EHOSTDOWN;

The server_unresponsive() timeout triggers cifs_reconnect(), which aborts
ongoing mid requests and causes the ls command to receive -EAGAIN, leading
to -EHOSTDOWN.

Fix this by introducing a dedicated `neg_start` field to
precisely tracks when the negotiate process begins. The timeout check
now uses this accurate timestamp instead of `lstrp`, ensuring that:

1. Timeout is only triggered after negotiate has actually run for 20s
2. The mechanism doesn't interfere with concurrent recovery processes
3. Uninitialized timestamps (value 0) don't trigger false timeouts

Fixes: 7ccc1465 ("smb: client: fix hang in wait_for_response() for negproto")
Signed-off-by: Wang Zhaolong <wangzhaolong@huaweicloud.com>
Signed-off-by: Steve French <stfrench@microsoft.com>