cifs: Simplify SMB2_open_init()

Reduce code duplication by calculating req->CreateContextsLength in
one place.

This is the last reference to "req" in the add_*_context functions,
remove that parameter.

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