cifs: set DFS root session in cifs_get_smb_ses()

Set the DFS root session pointer earlier when creating a new SMB
session to prevent racing with smb2_reconnect(), cifs_reconnect_tcon()
and DFS cache refresher.

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Cc: stable@vger.kernel.org # 6.2
Signed-off-by: Steve French <stfrench@microsoft.com>