[CIFS] use krb5 session key from first SMB session after a NegProt

Currently, any new kerberos SMB session overwrites the server's session
key. The session key should only be set by the first SMB session set up
on the socket.

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