cifs: try opening channels after mounting

After doing mount() successfully we call cifs_try_adding_channels()
which will open as many channels as it can.

Channels are closed when the master session is closed.

The master connection becomes the first channel.

,-------------> global cifs_tcp_ses_list <-------------------------.
|                                                                  |
'- TCP_Server_Info  <-->  TCP_Server_Info  <-->  TCP_Server_Info <-'
      (master con)           (chan#1 con)         (chan#2 con)
      |      ^                    ^                    ^
      v      '--------------------|--------------------'
   cifs_ses                       |
   - chan_count = 3               |
   - chans[] ---------------------'
   - smb3signingkey[]
      (master signing key)

Note how channel connections don't have sessions. That's because
cifs_ses can only be part of one linked list (list_head are internal
to the elements).

For signing keys, each channel has its own signing key which must be
used only after the channel has been bound. While it's binding it must
use the master session signing key.

For encryption keys, since channel connections do not have sessions
attached we must now find matching session by looping over all sessions
in smb2_get_enc_key().

Each channel is opened like a regular server connection but at the
session setup request step it must set the
SMB2_SESSION_REQ_FLAG_BINDING flag and use the session id to bind to.

Finally, while sending in compound_send_recv() for requests that
aren't negprot, ses-setup or binding related, use a channel by cycling
through the available ones (round-robin).

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>