xfrm: remove VLA usage in __xfrm6_sort()

In the quest to remove all stack VLA usage removed from the kernel[1],
just use XFRM_MAX_DEPTH as already done for the "class" array. In one
case, it'll do this loop up to 5, the other caller up to 6.

[1] https://lkml.org/lkml/2018/3/7/621



Co-developed-by: Andreas Christoforou <andreaschristofo@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>