CIFS: Enable encryption during session setup phase

CIFS: Enable encryption during session setup phase

commit cabfb3680f78981d26c078a26e5c748531257ebb upstream.

In order to allow encryption on SMB connection we need to exchange
a session key and generate encryption and decryption keys.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
[bwh: Backported to 3.16:
 - SMB2_sess_establish_session() has not been split out from SMB2_sess_setup()
   and there is additional cleanup to do on error, so keep the
   'goto keygen_exit'
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>