cifs: reinstate sharing of SMB sessions sans races

We do this by abandoning the global list of SMB sessions and instead
moving to a per-server list. This entails adding a new list head to the
TCP_Server_Info struct. The refcounting for the cifsSesInfo is moved to
a non-atomic variable. We have to protect it by a lock anyway, so there's
no benefit to making it an atomic. The list and refcount are protected
by the global cifs_tcp_ses_lock.

The patch also adds a new routines to find and put SMB sessions and
that properly take and put references under the lock.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>