cifs: Simplify SMB2_open_init()

We can point to the create contexts in just one place, we don't have
to do this in every add_*_context routine.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>