cifs: Split lanman auth from CIFS_SessSetup()

In preparation for splitting CIFS_SessSetup() into smaller more
manageable chunks, we first add helper functions.

We then proceed to split out lanman auth out of CIFS_SessSetup()
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Shirish Pargaonkar <spargaonkar@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>