[SMB3] Fix sec=krb5 on smb3 mounts

Kerberos, which is very important for security, was only enabled for
CIFS not SMB2/SMB3 mounts (e.g. vers=3.0)

Patch based on the information detailed in
http://thread.gmane.org/gmane.linux.kernel.cifs/10081/focus=10307


to enable Kerberized SMB2/SMB3

a) SMB2_negotiate: enable/use decode_negTokenInit in SMB2_negotiate
b) SMB2_sess_setup: handle Kerberos sectype and replicate Kerberos
   SMB1 processing done in sess_auth_kerberos

Signed-off-by: Noel Power <noel.power@suse.com>
Signed-off-by: Jim McDonough <jmcd@samba.org>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <steve.french@primarydata.com>