[CIFS] fix unicode string alignment in SPNEGO setup

Unicode strings need to be word aligned, but the code that handles that
is currently not taking the length of the SPNEGO blob into account. Fix
it to do so.

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